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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 6:25 am 
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Here's one which I think is a great choice. It's got superb frequency response, good isolation for safety, and multiple windings that you can use to combine two channels, adjust impedance ratio, etc:

http://www.newark.com/oep-oxford-electr ... gJgrvD_BwE

Datasheet here: http://docs-europe.electrocomponents.co ... 0fb25c.pdf

It's a little pricey, but the specs are very nice.

Connect it like this:
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You don't really need a transformer with two primaries like this, but since it has them, we'll use them to combine the channels. If you want even higher output, connect both secondaries in series (here I am showing just one in use, which is probably good enough).

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 11:50 am 
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Tom,

That’s a really good transformer. Price is not that bad either. Down to 30 Hz too. I might upgrade my converter.

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 1:52 pm 
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Tom,

Once again I am in debt to you. Thank you very much!!!. Looks like they cover the entire frequency response that I need. I will order two of them. In your diagram you do not show any resistors inserted inline with the connections. In past threads I noticed that everybody was inserting resistors on both positive feeds from the iPod device. Will they be necessary here and if so what do they do?

When I get one put together I will report back here with pictures and to report on how it sounds.

Thanks
BB


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 4:25 pm 
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The resistors are used to combine the L and R channels. Another way to do that is to use two separate primary windings like this transformer has. Then the resistors are not needed.

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 5:27 pm 
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'The OEP transformer Tom found should be a pretty good choice.

The primary DCR will sub for the mixing resistors, so they are not needed if you wire it as shown.

If it doesn't play loud enough, you can run the two secondaries in series to get a larger voltage step-up.

This part does have an isolation voltage rating. I'd feel pretty good using it with a transformer type set.

However, 1000V is not really high enough for use with an AC/DC radio, but would be better than no rating at all.

Ted

Quantity Price
1 $15.79

Electrical characteristics:

Winding ratio: 1+1:6.45+6.45
Impedance:
Primary 150 ohms parallel connected
600 ohms C.T. series connected
Secondary: 6.25k ohms parallel
25k ohms C.T. series connected
DC resistance (+/- 15%):
Primaries total: 20.6 ohms
Secondaries total: 1143 ohms
Inductance, measured at 1kHz, 0.27V:
Primaries 125mH min. per winding
Secondaries: 5.2H min. per winding
Proof voltage: primaries to secondaries: 1kV DC
Frequency range: 30Hz - 25kHz +/- 1.5dB.
Power: 100mW @ 300Hz and 1mW @ 30Hz
Distortion: less than 1% T.H.D. (ref.600 ohms)
30Hz - 25kHz measured at 0dBm
o Operating temperature range: 0 to +70 C
o o Storage temperature range: -25 C to + 120 C
N.B. Do not pass DC through windings

The size is about a 1 inch cube.


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 5:43 pm 
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It's true that the 1 kV isolation voltage on this part doesn't meet modern safety specs for commercially sold equipment. But this spec is certainly good enough to make sure that this transformer is not the weak link among the various tube equipment you have. If the spec on this transformer isn't good enough for you, then perhaps no vintage tube equipment of any kind is good enough for you. The power transformer isolation in lots of other tube equipment you have probably does not meet modern safety specs.

If you're comfortable plugging into an old audio amplifier with a transformer power supply, then you can be comfortable using a transformer like this.

In both cases, you should realize that it's not the same as modern equipment, and in the event of lightning storm or other event that causes big voltage spikes on the AC line, you can construct scenarios where a failure could occur, leaving the isolation compromised and not as safe thereafter.

I thought it was wise when people brought up the fact that little transformers that were likely designed as interstage transformers in transistor equipment might have surprisingly low isolation voltage specs (maybe as little as 50 or 100 V), where failure might be quite probable. But here we're getting into specs where we try to handle rare events, which is a different level of risk.

Good to be informed on what the tradeoffs and risks are, and then go ahead and use what you're comfortable with.

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 6:38 pm 
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Tom,

I ordered a couple of these transformers and it looks like I will get them in 3 business days.

In the meantime, what if I want to keep a stereo signal through this transformer to my German console. This German console is able to accept a 5 pin DIN plug for a tape deck. I have already been listening to a cassette deck through a RCA/phono to 5 pin DIN adapter and it works great in stereo except for the lack of volume. One of my other German consoles is not stereo so I think your diagram drawing is perfect for that one.

My plan would be to make a box for this transformer that has two RCA female inputs and two RCA female outputs. I already have the pig-tail adapters to go from RCA to 5 pin DIN. I also have a home-made adapter that has two male RCA on one end with the wires combined down to mono and a 3 pin DIN on the other end- this one is for my older German console that is not stereo. I also should have a pig-tail adapter that will convert the 1/8 inch plug to RCA.

Thanks,
BB


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Sat 24, 2018 8:19 pm 
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If you want to keep two separate audio channels for stereo, then two transformers are needed. Unfortunately there is no way around that. Use the same diagram as I made for you above, but there will be no connection to the second primary winding where I show the "R" channel connected.

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 28, 2018 3:21 am 
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Ok, I received the transformers today and they are pretty small with very small terminals for which to attach the wires to. First, I would like some clarification on just what side the wires should be connected to. The transformer has one side marked "primary" and the other side marked "secondary". And the pins are marked S1 S2 F1 F2. And there is a 9th pin sticking out of the bottom that does not appear to be marked on the label. Right now I am using a bluetooth transmitter to play my iPod. This is one of those things that plugs into the wall for power and then the little black box has two RCA female jacks that you use to plug into the back of a stereo receiver. It works fine on modern receivers and my old German radio using the correct cable adapter to change the RCA plugs to a 5 pin or 3 pin DIN jack but the volume is a little too low.

I jury-rigged some RCA cables together in such a way that I think you diagramed for me a few days ago in an earlier post. Does this look OK in my last photo? I know the connectors are sloppy, it is just the best I had for right now. So, how do I determine which side is connected to my iPod bluetooth player and which side goes to the receiver? I am not worried about stereo just yet, a mono set up will do just fine.



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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 28, 2018 3:33 am 
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Take a close look at the pinout diagram on the data sheet here: http://docs-europe.electrocomponents.co ... 0fb25c.pdf .

It looks like the connections may not be correct. Note on the pinout diagram that the first primary wing is the 1st and 3rd pin in the A row, and the second primary winding is the 2nd and 4th pin. This is somewhat counter intuitive.

Then connect your output to one secondary coil, for example the 1st and 3rd pins in the B row.

Primary windings go to your audio source, and the secondary winding goes to the audio input connections of your radio.

9th pin might not be connected to anything. Its main purpose is to define the orientation when looking at the connection pins so you can line them up with the diagram unambiguously.

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 28, 2018 4:09 am 
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Pin nine is the screen

Location (reference) designators...
A1 A2 A3 A4
B1 B2 B3 B4

as seen in Tom's pdf will identify the primary and secondary terminals.

Greg.


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 28, 2018 4:29 am 
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OK I just figured it out. I have sound that is much louder than without the transformer in line. While listening to rock and roll the mid-range frequencies sound a little distorted like they are being overdriven? Maybe turning down the volume on the IPOD will take away some of the distortion. The bass and treble sound good. It sounds like it is reproducing the entire frequency range. Right now I am playing it through a 1980's JVC receiver. Before I inserted this transformer inline, when I would switch from FM to the AUX input where this bluetooth thing is plugged into I would suffer a loss in volume of what I would call about 30%. Now with the transformer inserted inline the AUX input is actually a little louder than FM radio so this is exactly what I needed. I assume it will do the same for my old radios.

Tomorrow I will play with the volume on the iPod to see if some of this distortion goes away. I will try one of my vintage German radios tomorrow too.

In the meantime is there something that I am overlooking that is causing this distortion? For the primary side I used pin 3 and 4 to connect the left and right channels and pins 1 and 2 are the common connection.

What does that 9 pin do if it is a screen?

Thanks again,
BB


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 28, 2018 5:08 am 
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Don't run the iPod at its maximum volume. On my similar devices, they start to go into distortion a little at max volume. You might also be driving the transformer into saturation. So for both reasons, dial the iPod volume back to the point it is not louder than he comparable FM signal, and also make sure are a reasonable margin below the max volume on the iPod.

I guess pin 9 is basically a shield. You can try connecting it to the radio ground and see if has any effect. If you have no hum without it, I suspect it will have no effect.

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 28, 2018 2:24 pm 
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I took some time this morning to listen to it at different volume levels on the 1980's receiver and I think it is working perfectly. I turned the volume down on the iPod by about 15% and that seems to be the threshold where mid-range distortion disappears. I could not turn up the volume loud last night but this morning I was able to and I seem to have complete bass and treble response. Looks like this thing did the trick. I will try my German radios tonight.

Any ideas on what kind of connectors I should get that will make attaching the wires to the pins easy? Those pins are tiny and short. What about a circuit board of some type? I will make up a small box with the RCA jacks for signal in and out so that I can easily move the box to another radio.

Thanks Tom,
BB


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 28, 2018 3:01 pm 
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This device is specifically designed for direct PCB mounting on a 0.1inch grid.

Greg.


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 28, 2018 3:04 pm 
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Greg, I am very new to this vintage radio hobby and I certainly do not have any experience with circuit boards. Would it be possible to make a circuit board that this transformer mounts to that would allow easy hook up of the RCA jacks?


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 28, 2018 3:06 pm 
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search: 0.1inch Veroboard or 0.1inch Stripboard.

Greg.

Edit: search: RCA phono pcb mount.
jack and socket is also known as male and female.


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 28, 2018 3:29 pm 
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As Greg points out, a circuit board is one nice way to go. I’d suggest using a “Spark Fun” protoboard if you want to do that. But it’s also fine to simply attach your transformer to the inside of a small box with an adhesive like 5 minute epoxy, mount some panel type RCA jacks through holes in the box, and use jumper wires soldered to the transformer pins to make the necessary connections. Do what you’re most comfortable with.

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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 28, 2018 3:35 pm 
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I was afraid to solder of the ends of the wires to those little pins on the transformer for fear of overheating something inside the transformer. Would that be cause for concern? My original plan was to try to find small connectors that I could solder on to the ends of the wires and then push them on to those little pins on the transformer.


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 Post subject: Re: Pre amplifier for iPod input to raise volume
PostPosted: Feb Wed 28, 2018 3:40 pm 
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No concern. Those little pins are designed to be soldered.

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