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Old 8th April 2014, 11:16 AM   #1521
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Ten pounds of "stuff" in a 5 pound bag comes to mind looking at your BOSOZ, Terry.

If the load resistors (1K5) are the right value, and the feedback resistors are correct, the only thing I can think of that would cause the lower gain on the left is Q4/5 being way out of gain specification. You've got plenty of other pairs to try. Maybe the mighty ZM has other ideas.

Last edited by BobEllis; 8th April 2014 at 11:20 AM.
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Old 8th April 2014, 01:00 PM   #1522
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Hi Bob,

There is a little more room now because I replaced that huge transformer with two smaller ones. Imagine if I had used both power supplies, the step attenuator array and xlr connectors. As it is, I only have three input channels. I would still like to add a phono pre input.

Looking at the voltages on my schematic, it looks like the REF shunts are correct on both channels. I read about a 10.3V drop. R22/23 have a 1.6V drop on the right channel and 0.0V drop on the left. I guess that points to Q3/4. I will try replacing those first and see what happens. I'd rather not change the design if I don't have to. Those FET's are the matched units that came in the kit. I never checked them, just trusted they were right. I should probably go ahead and replace Q1/2 while I'm at it just to be sure.

Thanks, Terry
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Old 8th April 2014, 02:54 PM   #1523
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R22/23 should have no voltage drop. Perhaps you measured incorrectly. Mosfet gates don't draw current when healthy.

Look at each channel individually. The positive and inverting sides (or left and right sides of the schematic if you prefer) are well balanced. That points to normal operation. The only issue I see with the voltages you post is the left channel has low gain. Are you measuring without a load? (not connected to an amp) I still go back to low gain Q4/5.

Zen Mod where are you?
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Old 9th April 2014, 01:38 AM   #1524
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OK, so this is what I did today. First, I replaced Q4/5 on the left channel. That brought the DC voltages up to where the right channel's are. Good I think so I hook up the sine generator and test. With the sine generator output of 1.4vac, 1K, the Right channel outputs 5.09VAC, left channel 2.6VAC. OK, so even though the DC voltages look the same I go ahead and replace Q2/3. Well of course, no change. So now I need to figure why the output is low on the left channel even though the DC voltages all read almost exactly the same on all transistors. Done for today. I'll pull it all apart again in the morning and see if I can find something wrong at the inputs.

So a few questions if I may.

How much gain should this Preamp have? With a 1.4VAC input, left channel has 2.6vac out and right channel has 5.09vac output. Is it possible that the left channel is the one that is right?

I get output on both sides of the balanced output, even though the - side of the input is shorted. Is this normal?

The output jacks short the outputs if nothing is plugged in. Does this hurt anything?

What if only the - side of the output is shorted?

Thanks, Terry
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Old 9th April 2014, 02:16 AM   #1525
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Don't just randomly replace parts. Time to measure a bit more.

The closed loop gain should be 4 - set by the ratio of feedback resistors (39K) to input resistors (10K). Right channel is operating properly. The left channel either has too little open loop gain or is loaded too heavily to meet its gain. Did you double check that the left channel load resistors are actually 1K5?

Yes, the circuit acts as a single ended to balanced converter if the signal is unbalanced.

The outputs short if you don't connect anything? What do you have connected when measuring? Try disconnecting the preamp from the output jacks when measuring. Also measure the AC voltage on the other side of the output caps.

If you don't get normal gain with the output jack disconnected, check the AC voltage on the drains of Q4-5 with your signal input.

Hard to believe this simple circuit is so hard to troubleshoot.
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Old 9th April 2014, 08:49 PM   #1526
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Hi Bob,

I went through everything this morning. I double checked every resistor and all are correct. All but a couple of them are Deans so they all measure very close to the proper value. I did discover that D2 and D3 were shorted on the left channel. That may have explained why that channel was so different. Both channels are playing beautifully now.

I did have shorting sockets on all connections. I think that is good for the inputs but didn't think about that the output wouldn't like being shorted. I changed out the output sockets for non shorting type and added a couple RCA jacks for unbalanced outputs.

Thanks again for all your kind help. You make this hobby fun.

Blessings, Terry
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Old 10th April 2014, 12:04 PM   #1527
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I'm glad you found the problem and are enjoying your BOSOZ.
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Old 10th April 2014, 12:10 PM   #1528
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Glad to see it is working for you! I should finish mine soon. I have a question about the transformer you used. Is that an Antek transformer, and if so what part number is it?

Steve
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Old 10th April 2014, 12:40 PM   #1529
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Hi Steve,

There are actually two transformers there, stacked on top of each other. Antek part #AN-0215 & AS-0528. I put the 15V in parallel for the - rail and the 28V in series for the + rail. I had a large 400Va transformer in there originally that had dual voltage ouput of 55V and15V. It worked fine but was way over-kill and heavy. These do the job well and now I can use the big transformer for a new power amp.
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Old 14th April 2014, 11:40 PM   #1530
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Ok so I did some more reading through the thread and found this post.
post #99
where Metalman says to short the - output to ground for single ended use. That must be where I got the idea, I just don't remember. So, is it better to short the both - in and - out when using single ended?

Thanks, Terry
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