[JMS]

I have a stock META42 in a Hammond case, and it works fine without any audible hiss/hum using headphones. When I try to use this thing as a preamp I get a buzz that sounds like a 60hz hum, and also some hiss. Is this a ground loop problem, and how can I fix it? I'm using a fake ALPS pot. Does that need to be grounded? It's mounted onto a plastic face plate. Input appreciated. Thanks.

edit: Got rid of the buzz (I'm an idiot: was using tape-out from amp to META42 instead of direct from CD player). But, still a very audible hiss, which I don't get if I plug headphones in directly. Grounding pot didn't seem to help either.

 

[ppl]

With the Headphones still connected on the Output also connect to your power amp if the his is gine then place a load resistor across the Output of each channel of the META42. also a 100-220 ohm series resistor placed at R9 not R8 may also help. the Above is if the output stage is unstable due to a High Value of resistive load and the Cable capacitence.

The His is probaly from using an opamp that is noisy. and a device with lower noise is required. Since you gave no details about your circuit I can thus only offer the Above sweeping generlisations.

 

[JMS]

Thanks for the reply. I'm using 2x EL2002 output buffers fed by an AD8620 op-amp in a multiloop configuration. I jumpered across R8/R9 and normally I get no hiss even if I use low impedance headphones. I tried using a 75ohm resistor in series between the META42 and the power amp but it didn't help at all. I also tried a shorter cable, which also didn't help.

 

[JMS]

Forgot to mention that my power amp has an input impedance of 20kohms. Could this relatively high impedance compared to headphones be the difference?

 

[ppl]

did you try the amp connecterd to both the headphones and the power amp. Normaly the META42 should drive a power amp well and with the 8610 should be no hiss unless your power amp has so mutch gain that it dose not need a preamp. Put a 100 ohm resistor load on the output and still see if it still hisses also the value of R10 should be 150-200 ohms with the EL-2002's

 

[puppyslugg]

Hi ppl,

Your PM mailbox needs to be 'pruned'.

Sorry, JMS for hijacking you thread.