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KnightofAwesome 30th March 2011 05:12 AM

Gilmore Dynalo troubleshooting (Desperate)
I've decided to take my trouble here now instead of just AMB's help website, but this amp is giving me to much grief. I am absolutely out of (my known) options.
You can get the schematic here for the headphone amp:

Here is the help thread on AMB's forum (ignore the first couple of posts, the rest are significant): AMB Laboratories DIY Audio • View topic - High DC offset in the right channel

Now for some details:
The left channel works perfectly fine, the offset always stays below 5.0mV and it doesn't do any crazy offset spikes like the right channel does.
I've installed a zobel network on both channels consisting of a 22ohm resistor and a 0.047uf under AMB's advisory, but that hasn't fixed the constant switching of offset of ~5.0mV to ~400.0mV every couple of minutes.
I currently have the gain of the amp set to 3x (R16=2k R7=1K C27=33pF)

Any form of help would be massive at this point, This amp is driving (har har) me nuts.


singa 30th March 2011 06:05 AM

I've seen the schematic and read the communication between AMB and
you.AMB is expecting you know what you are doing but apparently you are not.From the high DC offset of the right channel tells you something is very
wrong somewhere.

1.Well for starters basic troubleshooting in DIY is to check
against the good channel whether all resistors and components are mounted
correctly and are of the said values in schematic.

2.Measure all relevant voltage drops to see if there are any huge discrepencies between the good channel.This will help to zero in on the
possible section of the circuit.

3.Check all pots to see if they are working and functional.

4.Lastly if you suspect oscillation then a scope will be useful.

Good luck. singa

artu 30th March 2011 06:13 AM

Your circuit has the DC servo or the manual adjusting resistors (R63, R64) option?

KnightofAwesome 30th March 2011 02:50 PM

Thank you singa and artu

1. I've checked that a multitude of times, I really wish there was a schematic showing what all the voltage drops should be on the board, I'll spend some time today and check all the voltage drops on the board, I'll post them here for reference.

2. Same as above, except the left channel will become the reference channel

3. Only one pot at the moment, and that's the log volume pot. That pot is working fine. I'll install any of the DC adjust when this whole ordeal is done.

4. I'll go by my electronics lab today sometime after lunch, I'll borrow a scope and see if i can (find oscillation) sort out the problem.

The DC servo is installed in the left channel at the moment, when the amp gets stable, I'll be doing all the adjusting necessary.


KnightofAwesome 30th March 2011 09:49 PM

I took a video of the scope when I was moving the volume pot, the steady line is the left channel, the line that's constantly moving up an down is the right channels output.

YouTube - Dynalo troubleshooting (scope)

I really scared that there's a problem with one of the jfets that came from AMB, since I'd have to replace all four.

I haven't measured all the voltage drops, but i did notice that when the offset goes haywire (~-400mV), the voltage drops changed across R18/R20. The voltages on the good board had ~1.45V across both, when the bad board has high offset the values change to about ~1.9V and ~1.1V, could Q3 or Q4 be fried?

Just a note, the offset on the right channel was never this large, It was like a switch had been turned on, and now it has this problem.

Thank you for taking the time to help out, I really really appreciate it at this point.

KnightofAwesome 31st March 2011 02:55 AM

I took the time to reheat a lot of the solder points on the board, the output transistors in particular, reconnected the power, turned it on... and it became stable although the offset was sitting at around ~20-25mV. I'll need to trim that down. I'm glad I got that worked out, it worries me that the high offset may come again...

Now, the thing that worries me, is when i turn the pot, the offset still moves a good +/-10mV with and without the servo. I'd like a theory as to why, does it have to do with the initial untrimmed offset?

It's nice to know that things are getting better, but I'm still worried, and my lack of electronics knowledge provides me without answers.

singa 31st March 2011 06:06 AM


I haven't measured all the voltage drops, but i did notice that when the offset goes haywire (~-400mV), the voltage drops changed across R18/R20. The voltages on the good board had ~1.45V across both, when the bad board has high offset the values change to about ~1.9V and ~1.1V, could Q3 or Q4 be fried?
Hi, Measure voltage drop across R11,15 (499 ohm) of good left channel
and see if the respective resistors at the right channel are the same ,if
not adjust R10,14 respectively.This should be your starting point reference
before you go on to the rest of the circuit.Also measure voltage across D1,2
led that it is 1.6V as stated.( If the voltage across R11,15 does not change
then measure with respect to common/ground or at the emitter of transistor,
I'm getting rusty at this).

Another thing do measure the good channel voltage at the Jfets and compare
with the faulty right channel.That should give you a good idea about the
situation of the front input section. Good luck. singa

johnferrier 31st March 2011 12:03 PM

maybe the potentiometer is bad? can you swap channels to see if the problem follows the pot? bad ground to pot? with amp off, measure the resistors . .. on or off, tap components to find an intermittent ? troubleshoot with the feedback loop open (original version has a gain of 35 open loop), it may be easier to find the problem (pull up one side of R16) . . . try circuit cooler (or a hot air gun) to isolate bad component . . . if you think jfets are bad, you could swap them left/right. it seems like several of the components (or connections) could cause this to happen, so it's difficult to isolate. good luck !


johnferrier 31st March 2011 12:10 PM

the original version uses component matching to fix offset, wo/adjust potentiometer



johnferrier 31st March 2011 02:35 PM

double check the vol. pot. with it off, measure the resistance from the input JFET to ground . .. make sure the resistance looks reasonable when you turn the pot. a connection can be bad (esp. to ground), a wiper can be dirty (try contact cleaner?), or the element defective (replace the potentiometer)


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