HRM Relay Popping - A Solution

['67_Plexi]

Several folks including myself have had issues with noisy switching on the dual master HRM amps. I finally have it sorted out and wanted to share what I did.
The largest sources of switching noise were due to DC levels on the contacts. These DC levels were due to leakage on the tone caps. The worst offenders by far are the ceramics with over 100mV of leakage. I had to try several before I found ones that were below 10mV, which I found to be the target upper limit value to keep the noise down. To measure the DC levels, just use a DVM from the tone control side of each tone cap to ground. Treat anything above 10mV as suspect. I found I needed to replace the mid cap on the clean channel to.

Here's the setup I used, which is virtually idential to HADs.

1. I used the drawings that came as part of Ayans posted pictures. The handwritten voltage doubler power supply is really wrong, but I did post a revised drawing on the same thread.
2. I used Fujitsu small signal 12V relays, having tried many I settled on these, plus they are physically one of the smallest.
3. I kept the switching ground seperate and lifted from the amp ground. This means using isolated footswitch sockets etc, HADs design.
4. I used a seperate 6.3V radio shack transformer, HADs filtering circuit and a 12V regulator. I just don't like using the filament cicuit for anything other than heaters. It's kind of like begging for trouble, even though I know it can be sucessful in some cases.
5. The LED's are switched seperately using the other contact to switching ground on a DPDT footswitch. This is supplied seperately from the 12V regulated supply through a 1K resistor, ala HADs design again.
6. I kept the relays on seperate boards, but placed as close to the relevant parts of the main board and pots as possible, once again thank you HAD.
7. Preamp grounding can also be a cause for noisy switching to, always worth a revisit.
8. Resistors to ground, across the contacts or caps accross the contacts didn't make any further improvements once I had fixed the underlying issues, so I omitted them.

The end result was a quiet as most commercial amps without using clamping circuits as Peavey did or going to LDRs. THe non-HRM's or single master models are not as prone to switching noise, but could still benefit from some of these suggestions.

Hope this info is useful to some.

Alan.

['67_Plexi]

One more thing, I also found that using short runs of low capacitance shielded coax on the grids of the first two preamp tubes helped keep switching noise down to. I know some people frown upon this, but the man himself did it so I don't feel too bad. You can offset any high end loss somewhat by using small value brite caps on the masters as needed.
I know it's not perfect, but it's all about trading things off to get a balance that is right for you.

[dogears]

I agree..... I use coax in these 4 spots as per historical accuracy.

One more thing, I also found that using short runs of low capacitance shielded coax on the grids of the first two preamp tubes helped keep switching noise down to. I know some people frown upon this, but the man himself did it so I don't feel too bad. You can offset any high end loss somewhat by using small value brite caps on the masters as needed.
I know it's not perfect, but it's all about trading things off to get a balance that is right for you.