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 Post subject: Solid state 1920's speaker amplifier revisited
PostPosted: Nov Fri 02, 2018 12:26 pm 
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I had built the following solid state amp for driving 1920's speakers maybe 2-3 years ago and I rarely use it at the moment.

Attachment:
Amp 12.png
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Figured it was time to revisit it to take care of a couple problems.

1. The low frequency response is not all that great even though I'm using push pull and the DC currents should cancel themselves out.
2. The output is limited to 50Vrms loaded with a 2.4K load due to that being the maximum voltage the transformer can handle.

A few things about the amp.

1. B+ can be anywhere from 20-40Vdc provided the 10K resistor is adjusted to keep the idle B+ current to 75mA.
2. The higher I go in B+ the higher maximum output I can get.
3. The no load output voltage can be much higher than the 50Vrms loaded voltage.
4. The diodes are resting against the chassis between the two power transistors with some heatsink grease to ensure they are coupled properly to the chassis far as heat is concerned as that keeps the amp gain more stable as the transistors heat up given the diodes are somewhat thermally sensitive.
5. The caps are used to eliminate fedback and seemed to work their best where they are conencted. Also all signal wiring is shielded which also helped to eliminate feedback.

Now to the issues at hand.

1. How would I increase the bass response as there's a larger high impedance 1920's speaker (uses cloth as the cone I think) that does have some decent bass response to it?

2. Would it be worth it to have Edcor design an output transformer that can handle at least 100Vrms with the same 600 ohm to 2.4K impedance?

3. How important is the DC resistance of the transformer windings that have DC passing through them?

Looking at the circuit the top darlington transistor draws 36mA and the bottom darlington transistor draws 39mA. Could that be enough of a current imbalance to cause the transformer to be lacking in the low frequency response or is it because I'm using push pull transformers that were never designed to have DC current through any winding?

My original thought was to find one of those 1920's speakers with better bass response then build a second amplifier and make a display for the four 1920's speakers I have with switches to select which ones conenct to the amp with the larger speaker serving to reproduce the bass down to 40-50Hz and the rest handling from somewhere around 200Hz on up. Would require a crossover though and I could build one that would work Said display would incluse an AM tuner (perhaps a good use for that MK-484 circuit) and an external input and maybe a bluetooth receiver.

That sort of display would be on down the road a bit as I really have no space to set up such a display which I would want large enough to accomodate at least 20 1920's speakers.

I do want the amp to be completely done though before I even think of such a display.


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Nov Mon 19, 2018 11:07 pm 
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So I decided to play around with the amp today and took some measurements. I then then thought wonder what would happen if I added negative feedback.

So I experimented with it and came up with connecting the collector of one transistor to the base of the other transistor through a 100K resistor or a 50K resistor and did the same with the other transistor.

I took measurements before I realized that the input voltage was changing with frequency so I have no actual measurements at this time, but will take some tomorrow.

The basics are this.

The frequency response improved quite a bit in the treble. Bass response with the 100K resistors is good to 40Hz before the sinewave starts to be altered and with the 50K resistors is good to 30Hz before the sinewave starts to be altered.

The voltage difference between the output with the 5K load and no load is much less.

I did have to readjust bias as I used no coupling caps in the feedback circuit, although I could easily add some. Might experiment with that tomorrow.

I tried taking the feedback from the secondary of the output transformer, but connecting any terminal of the output transformer to ground only caused feedback.

The amp itself is inherently unstable due to the high gain I'm getting from one transistor stage and all signal wiring had to be shielded along with a couple caps added to keep the amp from breaking into oscillation.

I think that if I were to build the amp on a chassis with more room so everything isn't so compact the amp wouldn't be as prone to unwanted oscillations.

As I recall I even had to wire one transformer the opposite way to keep the amp from breaking into oscillation due to inductive coupling even though the transformers are around 3-4" away from each other.

I really wanted to include the transformers in the feedback loop as that would be better, but I don't see a way I can do it without the amp breaking into oscillation.

I may try to ground one end of the output transformer secondary and connect the other end through a resistor to the center tap on the input transformer primary and see if that works.

If I get oscillation I can easily unground the primary of the input transformer and try it like that.

Here's the schematic showing the feedback resistors. The resistor values are not marked as I am not sure which value I will go with yet. While testing with the feedback I'll try removing the two caps and see if the amp is still stable.

Attachment:
Amp 13.png
Amp 13.png [ 33.49 KiB | Viewed 3050 times ]


Now I don't care about bass response given this will be driving 1920's speakers which have no real bass response except for that one larger speaker that I think used cloth as a cone or something similar.

Would love to find one of those.

I would then build a second amp and find a cabinet so that I can display my 1920's speakers and have them working with the larger speaker providing the bass to 40Hz.

The idea was to use a MK-484 one chip AM radio as the source with a simple two way crossover then each speaker would have a switch that would turn it on or off.

See I like the 1920's speakers and collecting them, but I see no point unless I have a good way to use them and get good sound from them. I've never been one to buy things unless I can use them.

Even if nothing becomes of this amp I will at least have the experience of what I learned from building and experimenting with this amp.

I should name it The Muntz as I'm trying to get proper operation with the least amount of components. :lol:



EDIT:

Here's the data

Attachment:
Data.png
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Next I'll try the feedback from the output to the center tap of the input transformer primary.

For some reason when I used a different scope that provides more data I got no oscillation when I grounded the secondary of the output transformer.


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Nov Wed 21, 2018 4:39 pm 
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I noticed something.

Across the 10 ohm resistor when I feed a 400Hz audio signal to the amp there's an 800Hz sinewave (happens at all frequencies) that is about 40-60mVpp which has a normal cycle then a slightly larger in amplitude cycle.

I can place a 470uF cap across the 10 ohm resistor and the waveform is reduced with the output jumping up by around a volt.

I'm assuming that waveform is due to the slight imbalance of the voltages on the transistor due to the DC resistance of the output transformer, but since the signals going to the transistor bases ate 180 degrees out of phase with each other shouldn't I be seeing just a 400Hz waveform?

Now to reduce that wabeform at 100Hz or below it could possibly take a cap that is over 2200uF.

Since the L shaoed chassis has screw holes on the small part that is bent 90 degrees I am seriously thinking about getting a piece of metal then removing all but the transistors and bias circuitry then moving them to the larger piece of metal which will allow for me to move things farther apart to reduce the chance of unwanted oscillation.

EDIT:

Due to the added feedback I was able to eliminate the two caps on the output transformer primary.

I experimented with a 470uF cap across the 10ohm resistor and that seemed to improve the frequency response somewhat and improved the linearity of the waveform near the transformer's 50Vrms rating.

Without the cap as the frequency went up the signal across the 10 ohm ressitor increased in amplitude.

I'll find a few different values of cap and see which value causes the signal to stay the same level down to 40Hz.

Here's the updated schematic.

Attachment:
Amp 14.png
Amp 14.png [ 32.37 KiB | Viewed 2992 times ]


The top transistor draws 36mA and the bottom transistor draws 39mA according to the voltages taken when I had B+ at 24Vdc.

Will need to take some new measurements with a B+ of 30Vdc which is what I use now.


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Nov Mon 26, 2018 2:58 pm 
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I tried a 5600uF cap and it seems to work well.

I found a sloped front metal project case I had bought a few months ago which was intended for something else. I might install the amp in it. If I do that I will add a 30Vdc regulator for the B+ and a power supply if I have room.

That way I can power it from an external AC transformer.

Tried to take voltage readings at 30Vdc B+ but the bais wouldn't remain at exactly 75mA as it would wander around 74.8-75.5mA slowly.

One problem is how I have the bias compensation done with the three diodes touching the metal chassis between the two transistors. That is a compromise as any bias change is delayed by the time it takes for the heat to travel to where the diodes are.

Here's what I'm thinking. Move the diodes to where one each is directly touching a transistor case then leave the third one where it is.

Only problem is it is working off the voltage change with temperature of all three diodes so will the two diodes changing voltage more quickly than the third really affect it? In a way I'm only guessing about the diodes being on the transistors based on what I saw in one amp where a diode was right on top of a driver transistor.


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Nov Mon 18, 2019 4:44 am 
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Just thought about this amp tonight.

Should I bother making the transistor currents equal or would there be no real benefit?

My thinking is by balancing the currents it would equal a net DC current in the output transformer of zero which would help the low frequency response some and may increase the maximum output before distortion.

I noticed the base voltage is slightly different as well by 4mV.

Now I think the easiest way to adjust the balance is by adjusting the value of the 1 ohm resistor which won't be easy given the low value.

That said should I try to adjust to the higher collector voltage or the lower collector voltage?


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Jan Wed 08, 2020 12:05 pm 
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Hi,
I think ,assume both primary each section have same number of turn , The DC for cancel out need to be equal to prevent core saturation. Even both side of primary may have different resistance.

For the bass response i think , it also related to the primary inductance , versus impedance. For example a 300V : 30V power transformer work at 60Hz versus 30V :3V at 60Hz although they have same ratio, the 30V :3V transformer cant work with 300V unless to increase the frequency up to 600Hz to prevent core saturation or maintenance a same flux density on the transformer core.. So in another view if using 30V:3V transformer for 300V:30V application , it will work well above 600Hz which can be the reason for poor low frequency response.


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Jan Wed 08, 2020 1:10 pm 
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The resistances are slightly different I believe.

That said the transformers used weren't designed for how I'm using them so not as much attention may have been paid to both halves of the transformers being the exact same resistance.

Not that important to have the bass response good to 20Hz as most 1920's speakers don't go nowhere that low in their frequency response.

Just wanted to be sure it was done right.


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Jul Wed 28, 2021 6:38 pm 
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Back to the amp.

Got bored and decided to do some tests on it.

B+ is 40Vdc and idle current has been adjusted to 75mA.

Interestingly enough the idle current may initially increase as the signal does, but as the transistors warm up the bias settles back to 75mA and that's with the amp driving a 5K load.

I used a 5,600uF cap across the 10 ohm resistor.

Here's the results with a 5K resistive load.

Attachment:
1 measure.jpg
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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Jul Wed 28, 2021 10:01 pm 
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Tube Radio wrote:
Back to the amp.

Got bored and decided to do some tests on it.

B+ is 40Vdc and idle current has been adjusted to 75mA.

Interestingly enough the idle current may initially increase as the signal does, but as the transistors warm up the bias settles back to 75mA and that's with the amp driving a 5K load.



One good way to help keep the bias and thermal stability better is to replace the diode array with a transistor and pot (a Vbe multiplier) in a package like a to-220 and mount it next to the two 2N6052's on the heat sink.

Since your output transistors are Darlingtons, it gives you the opportunity to do something interesting, that works well and results in a much flatter audio frequency response. One issue with the driver transformer can be peaks in the response, unless say the secondary or primary is moderately loaded. Simply the driver transformer is replaced with a choke, it needs to be about 2H inductance on each winding half, easy to wind on a small core with fine wire. Then the audio is coupled from the driver stage onto one side of the choke from a source resistance of around 6k. But if sufficient signal, you can omit the driver stage and just couple in the audio with a capacitor.

This method once a popular technique in Australian made car radios and it works well, provided the outputs are Darlingtons which reduces the drive power requirement, see attached circuit. (In this case the Darlingtons were made from a Silicon input transistor and a Germanium output transistor, the output transistors are electrically similar to the AD149).

It also has the Vbe multiplier. Notice the way transistors were sometimes drawn in the early 1960's.

Edit: If you want to dispense with the transformers, and you are fond of TO-66 cased Darlingtons, like I am, you could go for complimentary output devices. I found the attached circuit inside an aviation grade video monitor. It was for the vertical output deflection and it used both current and voltage feedback. It makes a really wonderful audio amp and it is also great in a servo system as a bidirectional DC motor driver. The OP amp is smiliar to a 741, which is starting to go deaf at 20kHz, so t does not have any high frequency stability issues. It does require split power supplies though.


Attachments:
AST1.jpg
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OPT1.jpg
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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Jul Wed 28, 2021 10:43 pm 
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Hadn't thought of using a transistor before.

So the collector goes to B+ the base to the pot connected to B+ and the emitter goes to the secondary center tap of the input transformer right?

However I do want to stick with the transformers as it keeps things real simple.

Plus it's for 1920s speakers and response doesn't need to be ruler flat.

Wonder if a CCS would be better to set the current and keep it at 75mA

The current even under load at full output will stabilize to 75mA as the transistors get warm but they only get a little above room temperature.

Think I may have a transistor I can use.


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Jul Thu 29, 2021 7:56 am 
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Attached is the classic design of the Vbe multipler, it was intended to create about two Vbe voltage drops between the bases of a complimentary output stage. But it can also be used in the case where you have a driver transformer, as shown in that transistor radio circuit I posted. One advantage over diodes is the transistor package is more easily screwed to the heat sink along with the transistors, so it better temperature compensates the transistors, because of the better thermal coupling, though you can always place bias diodes on the heat sink too, with thermal compound. For many low power amps, less than 1 watt, often the bias diodes are stuck somewhere "nearby" the output transistors. In higher power amps thermal coupling to the diodes or the Vbe multiplier transistor is advisable.
(The forward voltage drop of a diode or transistor's B-E junction drops with temperature, decreasing at about -2.1mV/Deg C)


Attachments:
vbe.jpg
vbe.jpg [ 39.27 KiB | Viewed 830 times ]
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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Jul Thu 29, 2021 1:35 pm 
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So I tried it like this. The current across the 10 ohm resistor increased as the output transistor temperature increases which is the exact opposite of what I need.

Attachment:
Amp 16.jpg
Amp 16.jpg [ 133.96 KiB | Viewed 822 times ]


I then tried it like this and there was not enough voltage drop across the transistor.

Attachment:
Amp 16-2.jpg
Amp 16-2.jpg [ 133 KiB | Viewed 822 times ]


Did I connect it wrong or will it just not work in this circuit?

Should I have connected it to where the transformer center tap goes to the collector with a series resistor between the collector and B+

Think I may go back to the diodes as they kept the voltage across the 10 ohm resistor at 750mV as the transistors heated up.


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Jul Thu 29, 2021 2:04 pm 
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Yes, you got it wrong, the circuitry is incorrect. You need to think of the Vbe multiplier as a 2 terminal device, with a specific voltage drop that appears across the collector and emitter.

If for example the two resistors leading to the base have the same value, the voltage is bisected, so the voltage that appears across the C-E is twice the base to emitter voltage. So it would be about 1.2V. If you change the ratio of the resistors, say make the one from the emitter to base lower than the one from the collector to base, then the voltage across the C-E will be higher. Hence the name Vbe multiplier. This is why the two resistors are often a potentiometer instead, with the slider connected to the base, to adjust the C-E voltage drop and set the output stage bias. The bias current via the Vbe multiplier in the bias chain has to be such that if the transistor was not there, the the voltage drop across the resistor from the base to the emitter connection has to exceed the at least one B-E drop, a few volts or so, so the transistor is definitely driven into conduction when its in circuit.


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Jul Thu 29, 2021 2:29 pm 
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Oh ok so it should be something like this right?

Attachment:
Amp 16-3.jpg
Amp 16-3.jpg [ 130.95 KiB | Viewed 817 times ]


It's a hard decision to switch from the diodes as the diodes do so good at keeping the voltage across the 10 ohm resistor at 750mV.

I'm amazed at how little B+ current the amp actually draws.

75mA total emitter current was the point where I found the highest gain and overall best operation.

At 5K load and 400Hz input frequency at 50Vrms output the combined collector current increases to about 77mA.

If I drop the load to 2.5K I have to increase the input by about 300mVrms for the same output, but the combined collector current only increases to about 83mA.


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Jul Thu 29, 2021 10:13 pm 
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Taken from the car radio design I posted, the circuit for the biasing (attached) you can vary the bias either with a resistor in series with the Vbe multiplier, in its ground leg, or with a pot rather than fixed resistors across the transistor, and you could eliminate the resistor in the ground leg.
(in the last circuit you posted the Transistor's C-E is connected across the 40V supply, which would destroy it, if it came into conduction)


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Vbe2.jpg
Vbe2.jpg [ 132.29 KiB | Viewed 808 times ]
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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Jul Thu 29, 2021 10:20 pm 
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I had forgotten a series resistor between the B+ and collector.

I might try that circuit tomorrow and see how well it works. Id basically have had the circuit you posted had I just connected the collector after the series resistor.

EDIT:

Didn't get to try the transistor.

Decided to see how much power the amp makes and doing a calculation of 50Vrms into 5K it is only 500mW, however at one point 2-3 years ago I tested it on a Peerless 7A reproducer and it was quite loud.


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Aug Mon 02, 2021 3:25 pm 
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So I tried the transistor again and here's what I got. It works well, although right at 750mV across the 10 ohm resistor, the adjustment is very touchy.

Attachment:
Amp 17.jpg
Amp 17.jpg [ 128.76 KiB | Viewed 703 times ]


EDIT:

I cured the pot sensitivity issue.

Put a 100K resistor in series with the lower end of the pot.

I also added a 160uF cap from the input transformer secondary center tap to ground so that the bias circuitry is bypassed for audio frequencies and I saw about a 1/2 volt increase in output.

I did notice that with the 5K load connected and an output I can increase the bias current and get more gain, but without a load it actually reduces the gain some.

Attachment:
Amp 18.jpg
Amp 18.jpg [ 135.41 KiB | Viewed 697 times ]


EDIT:

Took the amp home and connected it to the Peerless and the input is fed from my computer.

The power supply I have at home can only go to 29.5Vdc so I had to re-bias for that voltage.

Sounds very good.

Decided to connect my other two 1920's speakers along with the Peerless.

The amp drives them all easily.

Attachment:
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I do need to figure out a 40 volt regulator unless there's a readily available linear 40Vdc regulated supply.

EDIT:

Here's what I came up with for the voltage regulator.

Attachment:
Amp 19.jpg
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EDIT:

Here's a couple pictures of the amp.

Attachment:
11.jpg
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Attachment:
22.jpg
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I took four 1N5552 diodes and made a bridge rectifier. Soldered a 5600uF 80V cap across it. Looks like I'll need a power transformer capable of 40Vrms at 100mA minimum, but I'd prefer 200mA-500mA.

EDIT:

Here's what should be the final schematic.

Attachment:
Amp 20.jpg
Amp 20.jpg [ 188.02 KiB | Viewed 643 times ]


EDIT:

I made a box with three 10K pots and three SPDT switches.

The pots are in series with the middle terminal of the switches and one end terminal of each switch goes to the outer terminal of the transformer secondary while the other end terminal of each switch goes to the middle terminal of the transformer.

The speaker negatives go to the other outer terminal of the transformer while each positive speaker lead goes to the pot.

The pot doesn't reduce the output to each speaker by much (maybe 20K or higher pots would have worked better), but it does allow for a little adjustment so that all speakers are about the same loudness. The switches select whether each speaker is across the full or 1/2 transformer winding.


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 Post subject: Re: Solid state 1920's speaker amplifier revisited
PostPosted: Aug Sat 07, 2021 4:56 pm 
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Wasn't sure I was getting the full output from the amp so I decided to drive it from the balanced output of the pro audio sound card in my PC.

Originally I used an unbalanced cable and a 1/4 to RCA adapter to connect the amp.

In order to use the balanced output I had to make a cable using three RCA cables.

The RCA jack on the amp is connected to the amp circuit ground and the amp circuit has no actual reference to the power line ground which is the only way this actually works.

I took a 1/4" stereo plug to stereo RCA jack adapter then took an RCA cable cut it in half and used another end of a RCA cable. I took the signal wires of the two RCA cables and connected one to the signal wire and the other one to the ground wire of the third RCA cable.

What I'm thinking about doing if there's room is installing a 1/4" stereo jack and connecting the ground to the ring. That way when I want to plug in a balanced cable it will be fed a balanced signal and when I want to plug in an unbalanced cable the sleeve will contact the ring terminal and the amp will still work.

Now I may try and experiment with disconnecting the input transformer primary from the circuit ground and seeing if the amp still works right. My only concern is due to the high gain of the single transistor stage, it is very easy for the amp to break into oscillation.

When I first built the amp the signal wiring to the transformers and transistors was just regular wiring, but I could not get the amp stable until I replaced that wiring with shielded wire.

Also I had to wire either the input transformer secondary or output transformer primary out of phase as the amp would oscillate due to magnetic coupling.


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