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 Post subject: Low power stereo system
PostPosted: Apr Tue 12, 2011 12:03 pm 
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I was playing around with my V-M SEP 6BQ5 stereo amp which drives two 12" field coil Magnavox speakers and a pair of Isophon DP-19 tweeters and noticed the system doesn't have very good bass. One day while reading a thread on a Motorola three channel console I got the idea to add a bass channel to the V-M amp. I then decided to build an active low pass high pass filter. Was gonna put the filters in the amp itself, but decided to make the filters separate on their own chassis which would drive the two amps directly. For now one amp is a GEC 912 PLUS I built as few years ago and recently modified with a mixer stage to mix a stereo signal to mono.

I got a couple 15" Magnavox speakers from an Astro-Sonic consile that I will use for the bass. The plan is to scrap the current speaker boxes and build a couple open baffle boxes 34" H 18" W & 18" D to mount the speakers in.

The plan is to find a small 10-20 WPC stereo vacuum tube amp for the bass speakers so that the bass will be in stereo along with the rest of the speakers.

I will post the schematics here. Let me know if the circuitry will work properly as I have it.

I moved the driver tube from the control box that came with the V-M amp over to the filter chassis so that I can keep the control box free of any need for power and will only have the input jacks input selector vollume control and output jack. Right now I have no balance bass or treble controls (variable resistors left in place to hold knobs), although I do intend to find a 100K dual gang balance control like what is used on a lot of the better receivers which would work best for my application. I also may put a couple gain controls on the filter box for the bass and treble amps so that I can properly adjust gain, but I am not sure where to put them. Either 100K controls before the cathode followers or I can use 50K or less after the cathode followers which drive the filters.

Filter box

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V-M amp

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GEC 912 PLUS amp

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 Post subject:
PostPosted: Apr Sat 16, 2011 6:27 am 
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What is the audio source?

It sounds like experimentation and modification is in order. You are going down the right road... You can make your own cross over, buy parts or pre-assembled cross overs from say www.parts-express.com.

Of course the sub woofer is one speaker. As you know bass is not very directional. You may also improve your two way speakers with a better cross over or upgrade the tweeter.

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PostPosted: Apr Sat 16, 2011 1:02 pm 
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I might be able to improve things with a better tweeter, but I want to stay with the vintage theme for the speakers.


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PostPosted: Apr Sat 16, 2011 4:27 pm 
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Tube Radio wrote:
I might be able to improve things with a better tweeter, but I want to stay with the vintage theme for the speakers.


The first thing you need to do is come up with the specs you want to achieve. For your filters what are your desired cutoff frequencies and rolloff?

What is the overall gain you want from your filter box? Right now the gain stages (the 12AX7s with the 470k plate resistors) have a gain of about 72 each so the overall gain of the filter box is 72^2--- over 5000 or 74 DB. Do you really need all that voltage gain!

If I'm reading the values correctly on the schematic you posted, the high pass filter has a cutoff of about 150 Hz, and the low pass cuts off at about 15 Hz. Since they are both 2 pole filters the rolloff is -12 DB per octave for each.

Now I doubt you want the LP to cut off at 15 Hz right?
And I doubt you want the HP to pass down to 150 Hz.

So what are your desired specs, then from there you can design the filters to meet those requriements. Did you just copy the RC filter values from some other circuit? Why choose 10.7 k and 107k? Why not more standard values like 10k and 100k?

Always start with requirements first, then design to try and achieve those requirements. If you have trouble with design, let me know the specs you want and I can design it for you.

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PostPosted: Apr Sat 16, 2011 4:51 pm 
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The low pass cuts off at 148.8 Hz. Hard to tell on the schematic, but the low pass consists of a 10.7 K resistor a .1 uF cap to ground followed by a 107 K resistor and a .01 uF cap to ground.

I did plan for the low pass to be around 150 Hz as I use that as the crossover point in my home stereo system and it works good for the bass.

The tweeters and 12" speakers already have the crossover as they were used with the V-M amp before I decided to go this route. The gain of the stages will be adjusted as necessary once I get everything built. My thought is to add a couple variable resistors so that I can adjust the gain of the regular and bass channels once I get everything else set right.

The GEC 912 PLUS amp will eventually be replaced by a 15-30 WPC stereo vacuum tube amp which I may wind up building if I find a suitable chassis.

I am not too sure about the gain needed for the high pass amp with the driver tube for the V-M amp. Also are the cathode resistors in the cathode follower stages the correct values or should I go with a lower value resistor?

All component values on the active filter box except for the filters are what I think from experience will work, but are subject to change as I actually build and test the circuit.

Once I get enough money (maybe a few months away) I will buy the parts to build this and see what I get. Then I will make any necessary changes to the circuit. Would it be better to go with something like a 12AU7 for the cathode followers?


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 Post subject:
PostPosted: Apr Sat 16, 2011 6:32 pm 
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OK I had a hard time deciphering the values on the schematic,

For the LPF with 10.7k, 0.1 uf, 107k and 0.01 uf the cutoff is 150 Hz, and the HPF with 90 Hz. You would have about a -4 DB notch between 90 and 150 Hz with the LPF and HPF RC values you have now. I would put pots in the filter circuits to tailor the response to what you want; probably to get more overlap in the 90 to 150 Hz region.

As far as gain for the VM amp, if that uses 6BQ5s too, you only need about 8 Vrms to the control grids to drive them into clipping. You've got tons of gain before the driver so you could go with all 12AU7s and lower the plate resistors and still probably have more gain than you need for a line level signal source. From your VM and filter schematic, you have the feedback going directly to the cathodes of the 12AX7 drivers, which will be way too much negative feedback. You need a divider in there somewhere. Do you know about how much negative FB you want to use? If you have an idea of the FB you want then you can calculate the open loop gain of the driver/output stages.

I'd have to look at the whole picture in a little more detail to give you more definite answers about the open loop and closed loop gains. Need to know B+, output transformer specs, signal sources for sure yada yada.


Here are a few other observations by just looking at the schematic:

Can't hurt to use 12AU7s as cathode followers. Why waste the gain of a 12AX7 on a cathode follower?

I would also decouple the B+ for the various stages. Not good to run each stage off the same B+ you're asking for oscillations that way.

I would also put grid stoppers in series with the control grids of the preamp tubes to help eliminate RF pickup. You could also use 1k to10k grid stoppers on the control grids of the output tubes of the VM amp to reduce blocking and clipping distortion. Maybe some 470 ohm screen resistors too.

I'd have to do some calcs on the DC operating points of all the tubes to make sure they're biased correctly.

I'm of the opinion that less is more with amplifiers. Better to have the minimum of components for the desired result. With all these stages cap coupled and the filters, my hunch is you're going to have all kinds of phase distortion and possible instability problems with all this stuff. I could be wrong so don't take my word for it.

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 Post subject:
PostPosted: Apr Sat 16, 2011 7:03 pm 
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Good catch. I forgot to add the 10 K resistors in the feedback network.


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 Post subject:
PostPosted: Apr Sat 16, 2011 7:14 pm 
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One more thing I noticed is the feedback from the VM amp needs to go to the AX7 right after the filter network. The way you have it drawn it would provide positive feedback. You'd have a nice oscillator that way.

EDIT: On second thought I think the feedback you have drawn for the VM amp and driver is the correct phase.

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Last edited by Cdoose on Apr Sat 16, 2011 8:06 pm, edited 1 time in total.

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 Post subject:
PostPosted: Apr Sat 16, 2011 7:35 pm 
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Not sure if I calculated right, but the hpf uses the same components and crosses over at the same point as the lpf.

I will change the schematic and add the 10 K resistors in the feedback network when I get home.


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 Post subject:
PostPosted: Apr Mon 18, 2011 2:59 pm 
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I used the rc hp and lp calculators found here http://sim.okawa-denshi.jp/en/Fkeisan.htm to get the component values for the filters.

EDIT: I just found a second order filter calculator on that same site http://sim.okawa-denshi.jp/en/CRCRkeisan.htm and plugged in the values of resistors and capacitors I am using and got the same 148.7 Hz crossover frequency with the poles at 108.6 Hz and 203.8 Hz


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