Whammy headphone amp help

[Wang1]

Hi,
Not sure if this is the right place as i am new to this forum, i have just completed build of whammy by hardwiring, and when hooked up with phones, sound is flat, low volume, muddy and just nor what it should be. Phones are Hifiman 400i, which can be driven reasonable well with another, less powerful headphone amp i have



Obviously there is a problem somewhere.


The signal is coming through ok, and the background is absolutely dead quite, so earthing would not appear to be the problem.


I have checked the wiring many times, re-soldered all joints, replaced the opamp and mosfets (and put on separate board).
The power supply seems ok, +17v and -17v (leds), and on no load the will sit for hours without any heat, when i connect the circuit the voltage regulators will heat up slowly until sinks are very hot (too hot in my opinion), yet the mosfets are barley warm.


I have built a number of tube amps in the past without problem, but frustrated i cant seem to find the problem with this.


Any help would be greatly appreciated.


Thanks

 

[rayma]

Post some clear, close photos, and the schematic.

 

[Wang1]

I have attached photo but you wont see much from that due to the hard wiring, the schematic is the standard, i have not changed in any way.


Assuming the wiring is correct and no bad joints, what would cause such symptoms



Thanks

 

[MEPER]

Have you checked bias current by measure voltage over the 10 ohm source resistors?
DC voltage should be about 0.6V for 60mA bias?

 

[Wang1]

thanks, i will check and get back

 

[Wang1]

Yes, 680mv

 

[MEPER]

Assume that it is same poor sound in both channels?

It would be nice if you could look at the signal with a test signal as input but you don't have a scope?

To test if the amp can deliver some power you could check if it can deliver some voltage sving over a load resistor? .....maybe a 100 ohm resistor? ....a few volts RMS should be possible? ....5 volts RMS should be no problem?

 

[MEPER]

Then you can also check if the opamp gives the expected voltage gain?
Is it x4 or x5 .....something like this I think?

 

[Wang1]

ok, thanks for the tips, i dont have a scope anymore, where should i attach resistor

 

[MEPER]

The two 100 ohm load resistors should be attached from Left Out to Gnd and Right Out to Gnd. It will correspond to: Tip to Sleve and Ring to Sleve at the Jack connector.

 

[Andersonix]

Perhaps the PS is inadequate, so test it with an equivalent load (w same 100 Ohm power resistors) before connecting to the amp.

 

[Wang1]

Hi Meper,

with 100r connected across out/gnd, getting 10mV only, i tried 66r and get same.
100r across power supply +/gnd +17.6v, and -/gnd -17.5v.

Voltage measured across source resistors about 580mV,

Also attached voltages at opamp with dummy load resistors attached.

The outputs, pins 1 and 7 seem a bit odd

 

[Wang1]

attachment

 

[MEPER]

Since you have correct bias you should maybe concentrate on the op-amp circuit. It is this circuit that makes the voltage gain. So at op-amp output you should have x4-x5 gain compared to the op-amp input. Can you measure these two signal levels?

 

[Wang1]

not sure how to do that.


in any case i will remove boards and check all wiring again

 

[MEPER]

Output of op-amp is pin 7 for L and pin 1 for R channel. Both goes to a 47 ohm resistor (R17 and R27). You should be able to measure some Volts RMS at the op-amp output if you have a "decent" input signal and you turn up the volumen.