Background noise from headamp aristaeus

[qqiao]

I have a 2nd hand headamp aristaeus, the background noise follows the volume pot. I measured ac between L+/L- and R+/R-, the left is 3.48v when volume pot is turn to max while right is 1.74v, I swapped ecl84 and 12ax7 but still same value. Also I tried to adjusted vr p1, p2+,p3- but it didn't help, same value.

May I ask how can I solve the issue? Thank you in advance.

[qqiao]

I tried different ground connections, it looks like right channel connects to power gnd and left doesn’t.

So I removed power ground from amp chassis and connects pot to circuit ground, the ac level lowered about 2v and bg noise level is decreased accordingly. But when I touch the amp chassis, the noise level will be increased.

Now I always set the volume to maximum, there is no noise at max, I don’t know the reason.

[qqiao]

I did more testings and keep current setting, there are two gnd on aristaeus, one connects to inlet gnd and chassis connects to this gnd, one connects to power pcb ground. By default, pot connects to inlet gnd, but the noise level is very high, left/right noise is not same, right is lowered than left(1.8v/3.4v), I assume the power gnd is not that clean. I tried to adding a resistor(64k) between chassis to inlet gnd, the noise lowered, but it's not enough, still buzzing noise from my l300.

The second try is that disconnect chassis from inlet gnd and connects pot to pcb gnd. This time, left side is perfect, no noise at all, but right has higher noise level, I can hear small noise when pot turning up, but when it goes to a specific level and won't increase more with pot. But when I touch chassis, the noise level increases on right, left is good. 

How can I do further debugging? Thank you.

[purk]

Have u reached out to Justin?

[simmconn]

@qqiaothere are a quite few posts in the DIY forum of this site talking about estat amp grounding that you can use for reference. Basically I’ve learned the following:

1. The metal chassis should be connected to the protective ground on the AC power inlet. This is for safety, as well as a good starting point for a hum-free amp.

2. There are two types of ground on the volume pot, the signal ground and the metal frame/shaft ground. I would keep the signal ground running along with the input cables’ signal flow, and connect the metal frame ground to the chassis (usually by tightening the collar nut with a tooth lock washer is sufficient).

3. power-line induced noise can have a few different sources, such as due to different ground AC potentials between the source and the amp; due to the power transformer leakage flux flowing thru a closed signal or ground loop; or in tube amps, the leakage flux directly modulating the signal current in the tube. The common wisdom calls for single-point grounding, either at the power filter cap, or at the audio input jack. Note that the left and right audio cables can also create a ground loop, so it makes sense to run them closely to each other inside the enclosure.

4. Some units provide a “ground lift” feature, which gives the option to connect the electrical ground to the earth ground thru an RC in parallel as opposed to direct connection. I haven’t had personal experience where ground lift is especially useful.

Within the amp, everything is relative to the amp ground. The ground only becomes dirty when there is dirty current flowing from one grounding point to another. Your job is to find that current and get it out of your ground.

[qqiao]

Very straightforward,  I will follow the suggestion and do some tracing, thank you.

[qqiao]

Almost found the reason, the dc gnd and ac gnd doesn't have a connection, I put a 62k resistor and 0.1uf cap between them, almost no noise now.

The pot case connects to dc gnd, I found it's better than ac gnd. Thank you guys.

[qqiao]

@spritzer

Hi Spritzer,

Do you know the 12ax7 plate voltage and ecl86 grid bias voltage? Mine is 230v/-12v, not sure whether this is correct.  Thank you.

[kevin gilmore]

you cannot correctly setup this amp by measuring these voltages.

you need a signal generator and a scope.

leave the phase splitter adjustment alone and set the output plate voltages to about 320v. but this is only approximate.

what you want for the output tubes is maximum peak to peak voltage with symmetrical clipping.

what you want for the phase splitter is balanced peak to peak voltages between the 2 output tubes.

 

[justin]

On 2/12/2023 at 2:39 PM, qqiao said:

Almost found the reason, the dc gnd and ac gnd doesn't have a connection, I put a 62k resistor and 0.1uf cap between them, almost no noise now.

The pot case connects to dc gnd, I found it's better than ac gnd. Thank you guys.

depending on how old the amp is, we added a ground loop breaker later, but where did you come up with this 62k value? We use 10 ohms in parallel with the .1uF cap (300VAC mains rated) in parallel with a 35A bridge rectifier

you can also try grounding one side of the filament supplies, which are AC

Surprisingly, we get only 10mVrms of "noise" on the output after careful selection of 12AX7/ECC83 tubes, which is going to have the most influence on your hum. Best I found were the JJ ECC83S which shipped with the amp. Old "special quiet tubes" may not be so quiet.

Edited February 23, 2023 by justin

[qqiao]

Hi Kevin and Justin,

Thank you very much for the invaluable information.  The power supply is made by myself and 62k is just from my inventory. With current setup, there is no noise from output, and Justin is correct, I tried some NOS tubes and the best one is JJ.

With respect to adjustment, I think I need more study to do it.