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Old 27th April 2014, 04:17 PM   #721
Preamp is offline Preamp  Germany
Islander
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Just did the measurements again. Used a Multimeter and left the amps powered on for 3-4 seconds only, since I don't have a heatsink mounted.

R16/R17: 410mV over 4R7 ~ 87mA
R13/R14: 1.05V over 18R ~ 58mA
R9/R10: 10V over 220R ~ 45mA
D1-D3: 2.57V

Both channels measure the same within a couple millivolts.

Rails:
15.44V @ 0.37A
-14.8V @ 0.37A

According to simulation (no exact models though), the current through R9/R10 seems a little high, which likely biases the output stage higher. Do you think this might be due to the 2SJ103/2SK246?
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Old 27th April 2014, 04:52 PM   #722
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I don't think there's any problem in the circuit, just the discrepancy between the calculated Ibias from the voltage across the Re's (about 160-170mA for two channels) and the indication on the psu meter IF you measured across R11, R12. THAT is the voltage that determines the bias current. Did you measure that?

jan
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Last edited by jan.didden; 27th April 2014 at 04:55 PM.
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Old 27th April 2014, 05:05 PM   #723
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Just did it: 640mV over 4R7...
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Old 27th April 2014, 05:41 PM   #724
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Quote:
Originally Posted by Preamp View Post
Just did it: 640mV over 4R7...
... which is 136mA (if it is across a single 4.7 ohms); two channels is then 272mA. Getting closer....
Maybe you should put the transistors on a heat sink. Keeping them cooler will lower the bias current, and you probably get into the ballpark.

Or else check with Richard (he's a member here) if his '>200mA' includes 270mA or so...

Jan
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Old 27th April 2014, 05:59 PM   #725
Preamp is offline Preamp  Germany
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The transistors didn't get hot during the short measurement period, only a little warmer than room temperature. I'll report back how it behaves once it's heatsinked.

I hope Richard still follows this thread and chimes in .
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Old 1st May 2014, 09:32 PM   #726
ammel68 is offline ammel68  United States
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I just finished building this Marsh design and I can not get it to work properly.
I'm trying to use it as preamp. instead of a headphone amp. I purchased both the board and semiconductors from the TechDIY store so I wouldn't have to fool around with matching transistors.
All the components are new and I checked and double-checked each component, except for the diodes, before soldering everything in place.
The only thing I changed was C1 and C2 to 470uF.

When I apply power(+-15V), all the output devices' heatsinks get nice and toasty in a couple of minutes and most importantly there's no smoke.
When I connect a DVM across the outputs, I get 0 volts on both channels even when switching to the 200mV setting...no offset at all. Turning the 1K trimmers either direction doesn't change anything. I have tried measuring the offset with the input shorted and not shorted, but I still get nada.

So I decide to connect it to an amplifier. A few seconds after I power it up, R12(4.7 ohm) starts smoking.
I quickly cut it off, disconnect the RCA's from the amplifier and put the amp back on my bench. After applying power R12 isn't smoking anymore. It now runs cool like all the other 1 watt resistors.
So for whatever reason...R12 only smokes when the output is plugged into an amplifier.

I have measurements across R11-R17 if someone here needs them for troubleshooting.
Output transistors are 2SB649AC and 2SD669AC instead of the MJE devices.

I'm not sure what's going on here, so any help would be GREATLY appreciated!

Thanks!
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Old 2nd May 2014, 08:01 AM   #727
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Did you measure your +-15V under load? Does any one of them drop significantly?
No short circuits at the heat sink or anywhere else?
Do you have a scope to look for oscillations?
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Old 2nd May 2014, 04:12 PM   #728
ammel68 is offline ammel68  United States
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Quote:
Originally Posted by Preamp View Post
Did you measure your +-15V under load? Does any one of them drop significantly?
No short circuits at the heat sink or anywhere else?
Do you have a scope to look for oscillations?
The only things(RCA interconnects going to and from the board) I assumed as being good, turned out to be the problem.
Apparently they were shorting out the outputs and causing some of the emitter resistors to smoke when placed in certain positions or moved a certain way.
Even though they were new and not particularly cheap interconnects, it just goes to show you can not assume anything when working with electronics.

My Marsh amp is now working as it should and sounds great!

Also, thanks to John at TechDIY for the nice PCB's and matched transistors.
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Old 2nd May 2014, 07:10 PM   #729
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Originally Posted by ammel68 View Post
My Marsh amp is now working as it should and sounds great!
Nice
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Old 2nd May 2014, 10:47 PM   #730
Preamp is offline Preamp  Germany
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Quote:
Originally Posted by jan.didden View Post
... which is 136mA (if it is across a single 4.7 ohms); two channels is then 272mA. Getting closer....
Maybe you should put the transistors on a heat sink. Keeping them cooler will lower the bias current, and you probably get into the ballpark.
The amp is heatsinked now, but the rail current only drops to 0.35A from 0.37A. I ran it for about 10 minutes and all the TO92's got a little hot to the touch. Guess this is not normal .
Out of curiosity I shorted one of the three diodes, which resulted in a bias current of about 47mA instead of 136mA (rail current was lower, too). IMHO this looks like I'm either 40mA too high or too low .

I thought about replacing the three diodes with a single red LED. They measured 2.58V across, which is a little odd 0.86V per diode. Roughly 2.1V seems to be the better fit here. What scares me are the 10V across R9/R10 (220R in my case), which should mean something like 40mA through the diode string. Will that fry a single LED?
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