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Reparing Class A symetrically driven Headphone Amp kit
Reparing Class A symetrically driven Headphone Amp kit
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Old 30th June 2020, 04:14 PM   #1
Lawrence.M is offline Lawrence.M  United Kingdom
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Default Reparing Class A symetrically driven Headphone Amp kit

I am trying to repair my Linsley Hood designed Class A symetrically driven headphone amp section of my pre-amp that I use as a high level output to my Power amp.
I am guessing that the output transistors are faulty(after over 10 years of use) so have removed them.These are the TIP 41C and TIP 42C(higher rated than the original design which shows TIP 41B and TIP 42B). My Question is,is it important for these output devices to be matched to have close hfe values,particularly as they get quite warm and I assume they would last longer if they shared the output current evenly.If this is important where can I get matched pairs from?
Alternatively I have a lot of quite closely matched(eg-197 hfe PNP/170 hfe NPN) MJE 15034 and MJE 15035 transistors) to hand which have similar specs to the TIP 41C and TIP 42C but slightly lower maximum current and Power ratings(4 Amps,350V,50 Watts as opposed to 6 Amps,100V, 65 Watts) according to the data sheets.Would these be a better alternative?
I have attached the schematic that came with the kit for reference.
Any advice much appreciated.
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File Type: jpg Class A Symetrically driven headphone amp3.jpg (541.0 KB, 107 views)
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Old 30th June 2020, 05:08 PM   #2
rayma is online now rayma  United States
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The single positive and single negative output transistors automatically have the same DC bias current.
If paralleled devices for each polarity are used, those should be matched.

You didn't mention the actual problem it has.

Last edited by rayma; 30th June 2020 at 05:18 PM.
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Old 30th June 2020, 08:23 PM   #3
Lawrence.M is offline Lawrence.M  United Kingdom
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Quote:
Originally Posted by rayma View Post
The single positive and single negative output transistors automatically have the same DC bias current.
If paralleled devices for each polarity are used, those should be matched.

You didn't mention the actual problem it has.
Thanks Rayma,One channel would cut out every so often,then eventually there was no music and just a buzz/Hum sounding like mains hum.To complicate the issue,when the channel first cut out I found that the off board power supply to the pre-amp had faulty regulators.I replaced these and the problem came back but only when using the high output from the pre-amp which uses the class A headphone circuit.As the output transistors get quite warm and the problem was initially intermittent,I thought they might be the culprits.Also when looking at the board there were no obvious signs of component damage like bulging caps etc.
After taking out the TIP41C's they measured an hfe gain of 55 and 53 which seems low at least compared to the TIP42C's that measured 187 and 203.
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Old 3rd July 2020, 04:04 PM   #4
Lawrence.M is offline Lawrence.M  United Kingdom
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Default DC Offset seems way to high

Ive just replaced the output transistors but the DC offset is measuring 0.77V and 0.69V from each channel and the trim pots don't seem to make any difference throughout there adjustment range. Any One got any ideas of where I should look next?
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Old 4th July 2020, 05:27 AM   #5
PRR is offline PRR  United States
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Reparing Class A symetrically driven Headphone Amp kit
Yank R9. Ground the input. Measure all the key voltages. I've sketched approximate guesses. Yours would be similar, but I expect you will find one/some WAY off. Crayon your readings and post back.
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Old 4th July 2020, 08:54 AM   #6
Lawrence.M is offline Lawrence.M  United Kingdom
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Default Testing ciruit

Quote:
Originally Posted by PRR View Post
Yank R9. Ground the input. Measure all the key voltages. I've sketched approximate guesses. Yours would be similar, but I expect you will find one/some WAY off. Crayon your readings and post back.
Thanks PRR.But before I start what do you mean by Yank R9?
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Old 4th July 2020, 07:37 PM   #7
sgrossklass is offline sgrossklass  Germany
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You're supposed to (temporarily) remove it.
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Old 4th July 2020, 09:57 PM   #8
Lawrence.M is offline Lawrence.M  United Kingdom
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Ah yes I see.
Thanks sgrossklass
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Old Today, 05:37 PM   #9
Lawrence.M is offline Lawrence.M  United Kingdom
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Default Measurements compared to ideal

Here are the measurements taken for comparison.The Green are one channel and the blue are the other one.They seem to be pretty close.I don't know whether the 1.5V and 1.47V DC offset at the output is reasonable?If so strange that I couldn't adjust this to lower than 0.77V and 0.69V with the trimpots before temporarily removing the resistors for testing.
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File Type: jpg Amp measurements2.jpg (202.8 KB, 0 views)
 

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