amp picks iup radio transmission

[h_andree]

Hello all,

I have problem. I've put together a 4 channel gain clone
2 lm3875s in paralel with input buffer DRV134. Voltage
regulators. MUR 1520s rectifiers. Different transformers for
input stage and power. Everything is maintained
in the case.
The amp plays wonderful.
No noise. Excellent. However, I've had 2 occurances
where my amp picked up the radio of a cab driving by.

My friend has build an identical amp and has the same problem.
The occurance is infrequent so difficult to test.

I never happened for myself or my friend when we had the
SE tube amps. The gain is high: 36 dB. The case is
aluminum shielded (par-metal.com). My friend claims it
even happened one time when he had the amp muted
(Shorted input to ground!).

I had the same problem with an old Rotel receiver in the past.

Obviously we have a design issue. What should we be looking at?

Thanks

Harry

[pinkmouse]

Dodgy interconnects?

[tiltedhalo]

These chips the entire lm38-- and 40 are all very high speed wide banwidth amplifiers up two 16MHZ, the drop off is drastic, but they will amplify low rf and ultrasonics. They are also are very intollerant of input ground loops they will also create oscillations. Things to help-
Are your inputs tied to ground IE. 1Kohm input resistor 22Kohm to ground? make sure this is down at the input to each chip.
Try a 220pf cap on the input to ground, you might have to play with values 100pf to 220pf range.
Are all you input voltages bypassed with .1uf caps to ground, if not put them as close as possible to the chips power input pins. This should get read of you rf problems.

[Nordic]

I think the datasheet recommends a 220pf cap between input pins for just that.

[bostonflyer]

Hi Harry,

I had the same problem. All I did was wrap all the input and output internal wires with aluminium tape. I made sure the tape was grounded to the chasis - no more rf.

Bob