[MrJ]

I've had my Dynahi running for 6+ months now without any problems and have been relishing in the amazing quality sound. I had noticed that recently (only last couple of weeks) there was a high pitched sound as I turned on the amp, so would turn it off immediately, wait a few secs then turn it on again and all would seem to be well. But recently I turned on my Dynahi with my SR325i's plugged in and volume down to low as usual practice, and there was a short high pitched sound, followed by a pop and then a burning plastic / electrical smell. I turned everything off immediately in horror. Later I tested the amp with some cheap headphones and it was fine, so tried the Grados again, and they then sounded soft, dull, and the right ear had a buzzing sound so have sent them in for repair. My Senn's and cheaper earbuds etc still sound fine and show no sign of the problem, and the amp shows now sign of damage. Any ideas what could have happened and how to prevent this from reoccurring?

 

[Pars]

Quote:

Originally Posted by MrJ /img/forum/go_quote.gif I've had my Dynahi running for 6+ months now without any problems and have been relishing in the amazing quality sound. I had noticed that recently (only last couple of weeks) there was a high pitched sound as I turned on the amp, so would turn it off immediately, wait a few secs then turn it on again and all would seem to be well. But recently I turned on my Dynahi with my SR325i's plugged in and volume down to low as usual practice, and there was a short high pitched sound, followed by a pop and then a burning plastic / electrical smell. I turned everything off immediately in horror. Later I tested the amp with some cheap headphones and it was fine, so tried the Grados again, and they then sounded soft, dull, and the right ear had a buzzing sound so have sent them in for repair. My Senn's and cheaper earbuds etc still sound fine and show no sign of the problem, and the amp shows now sign of damage. Any ideas what could have happened and how to prevent this from reoccurring?

Do you have a voltmeter (DMM)? Or a friend with one? You need to check the DC offset of the amp output. To do this, you need to measure the L output to ground, and the R output to ground (DC Volts). The "short high pitched sounds" are definitely not normal, nor are burning drivers. Perhaps if you know who the builder is/was, you could contact them and have them fix it?

 

[MrJ]

Yes, I've checked the DC offset of both channels and both are very low, almost undetectable using my DMM in the mV range. I built the amp, and was very careful to make sure DC offset was tuned as close to 0 as possible whilst biasing the channels and before installing the servo. I did also test that the servo was working bringing any dc floating back to zero.

I think the problem was that there must have been a surge on startup into the headphones. The cause of this surge is yet to be found. I've opened up the amp box (power and amp in separate boxes) and noticed that the left channel is draining almost all of the (-)ve voltage down to a few volts whilst the (+)ve voltage is a stable 30V. If I disconnect power to the left board the voltages for the right board go back to a stable (+/-)30V and 0V. Something must have happened or shorted between the time I completed the build and now on the left channel to cause this. I can't see any obvious signs of burnt out components or shorts on the top of the left board, but will pull it out over the next couple of days and see if there are any shorts or problems on the bottom of the left board.

Although it looks like there is a fault in the left channel board, the burning smell was coming from the right channel of the headphones and this channel was also the one that started buzzzing after the event. I'll do some more investigation into the suspect left channel and post my findings.

 

[tomb]

Doesn't the Dynahi use DC servos? If so, perhaps the high-pitched sound was the opamp oscillating in the servo - which might also mean that DC offset went out of control for a short period on startup. I have heard that the DC servo can sometimes get squirrelly on the Dynahi-style amps.

 

[majkel]

It would be a miracle when a DC servo started to oscillate.

It's bandwith is no higher than several Hz's. I confirm, there must have been an oscillation causing the output stage go out of control and either high DC appeared or high amplitude ultrasonic oscillation overpowering the headphones. Even if you use DC servo, it won't protect headphones against current surges because DC servos are very slow. I guess you should verify all solder joints and correct the suspicious ones as a poor contact may cause wrong transistors biasing and oscillations or DC on output as the result.

 

[DigiPete]

What gain do you have your Dynahi set at?

If it is too low and you have a lower impeadance headset plugged in
it could get unstable.

 

[nikongod]

because of the sub-1hz speed of the DC servo in the gilmore amps, the servo can take a few seconds to do much of anything.

i would check up on the amp's dc offset without the servo in. the amp should not drit if tis properly set up, but then lots of stuff should not happen that does from time to time.

 

[MrJ]

Quote:

Originally Posted by DigiPete /img/forum/go_quote.gif What gain do you have your Dynahi set at? If it is too low and you have a lower impeadance headset plugged in it could get unstable.

The gain is set to 12

Ra=1.21K
Rb=100

 

[DigiPete]

Try posting at headwize in the DIY/Dynahi amp thread