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Old 29th June 2015, 03:56 PM   #1
Shelah is offline Shelah  Zambia
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Default output voltage at speaker terminals

good evening DIYers,

i just made an 2 channel amplifier with complementary bjt output. i just have one problem. channel 2 is giving 30mv and the channel 1 is giving 238mv at the speaker terminals. after i counter checked the connections and components of channel 1 with channel 2, and all was well. i tested for the second time and still got the same 238mv on channel 1. now i read an article that was produced by Rod Elliot. He said that any output voltage below 100mv is fine and any above that is exessive even though some amplifiers are seen to work well at output voltages exceeding 100mv. i tried changing the bias voltage buh still there was no change. what could be the problem?

Last edited by Shelah; 29th June 2015 at 04:01 PM.
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Old 29th June 2015, 04:39 PM   #2
llwhtt is online now llwhtt  United States
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That's a hard question to answer without knowing what amplifier it is. Can you post a schematic? Bias has nothing to do with DC offset. Does the front end have a differential transistor pair?

Craig
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Old 30th June 2015, 08:11 AM   #3
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I would imagine that's low enough anyway, but as already suggested post the schematic - it's normally pretty easy to add an adjustment (or you might just need to swap a couple of transistors round).
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Old 30th June 2015, 08:51 PM   #4
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Most amplifiers have a trimmer for the dc offset but some have fixed resistors.
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Old 1st July 2015, 04:57 AM   #5
anatech is offline anatech  Canada
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Hi Shelah,
This sounds like a classic case of input transistors that were not matched. If your amplifier uses a differential pair of transistors for the input stage, it is very important that they are matched in Beta. This stage compares the input signal with the output signal and performs subtraction to generate a correction to reduce the distortion. DC offset is just one form of correction the input stage performs.

One thing you can read into what was just said is that your distortion performance is much improved with a matched pair of transistors. This is a true statement.

One other thing can contribute to higher DC offsets. Low beta transistors in that differential pair will have higher base current (if they are BJTs). This current drops voltage across the resistor between the base and signal ground, subtracting from the inverting side. The voltage between the inverting transistors base and speaker output (Assuming there is a DC blocking capacitor between that base plus resistor and signal ground). Mismatched resistor values will exacerbate differences in base currents.

It can be difficult to match transistor beta because transistors are very sensitive to temperature. In order to test the beta, the different transistors will need to be at the same temperature. All you can do is test and leave the parts in there long enough to stabilize their temperature. Once you have a matched pair, they need to be held in contact with each other to maintain the same temperature across both of them.

Sorry for the long post, but it's important to know the why and how as well as the reason for your DC offsets. The numbers you posted are quite common for mismatched transistors.

-Chris
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Old 1st July 2015, 06:38 AM   #6
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A schematic can be very helpful.

In your negative feedback, do you use AC grounding or you do DC negative feedback. If you use DC negative feedback, you have a lot of DC gain, any mismatch at the input LTP will be amplified.

If you use complementary IPS, it is possible to have that kind of offset even if it is AC negative feedback. In fact I was just mentioned to Mr. Bob Cordell in his thread two weeks ago and he acknowledge it can happen. The problem is not even mismatch of the LTP, it's the mismatch of the tail current of the two complementary LTPs. Go to Mr. Cordell's thread, back like 6 pages and you'll see the discussion.
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Old 1st July 2015, 09:46 AM   #7
Shelah is offline Shelah  Zambia
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this is the schematic i only made the class ab section, no h step.
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File Type: pdf ExpressSCH-1.pdf (54.3 KB, 69 views)
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Old 1st July 2015, 09:51 AM   #8
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Does the set zero adjustment have any effect? Is the + and - 160V correct?
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Old 1st July 2015, 11:17 AM   #9
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The front end transistors, did you match them.

Gajanan Phadte
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Old 1st July 2015, 12:20 PM   #10
anatech is offline anatech  Canada
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Hi Shelah,
In this circuit where you have complimentary diff pairs, matching all four (very time consuming, need lots of transistors to pick from) makes a big difference in performance. Any imbalance of Beta will cause DC offset. So, get all four as close as you can.

This general form of input was popular with SAE and Phase Linear just to name a few. Many years of experience with these has taught me that there is no other way to make these sound great other than doing it the right way. I must have bought thousands of those transistors (2N5551 and 2N5401). I still have over 100 of each in order to find matched quads. Get the like transistor types as close as you can, then find the closest matched other pair. Expect to achieve about 5 mV DC offset, <10 mV really if the diff pairs are properly matched. This also improves the sound quality of the circuit.

Hi Alan0354,
Unequal base current is another way of saying non-matched Beta in these circuits. Attempting to unbalance a pair might help with DC offset, but will hurt in the distortion department.

-Chris
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