Technical Assistance/Advice Thread

[MLA]

I usually attach pot ground to star ground directly. Is that one of the options you already tried? 

Also, are your inputs tied to star ground as well, or to the pot?

[gepardcv]

Yes, I've tried pots to star ground. No change.

Input grounds are connected to the chassis (using the Neutrik connection pin), and not to the pot or star ground. I read that it's supposedly the Right Way in the famous Rane Note. (Unless I brutally misunderstood what I read).

[Laowei]

My amp is dead quiet. In my 2 case GG, I tie PS and Amp grounds to a single star ground on the Amp chassis. IEC ground is connected at the PS chassis and then carried separately to another point on the Amp chassis. My input is the same as yours, with XLR pin 1 grounded to the chassis, but I leave the pot's ground left floating (with each L and R  +/- signal ground connected for the attenuator to work). Being balanced, they will null.

With your pot ground connected to the amp boards and then daisy chained to the PS board, possible ground loop there. Try disconnecting and see if the hum is still there. 

https://www.tortugaaudio.com/articles/humming-along-to-the-pin-1-problem/

 

Edited August 20, 2017 by Laowei

[mypasswordis]

Did you make a chassis ground on the amp, tie that to amp star ground and psu star ground which is connected to psu chassis ground?

Edit: wow this is a great pic

Edited August 20, 2017 by mypasswordis

[Whitigir]

I have pot ground to chassis and there is no hum.  Is it hum or hiss that you are hearing ?

[MLA]

3 hours ago, gepardcv said:

Yes, I've tried pots to star ground. No change.

Input grounds are connected to the chassis (using the Neutrik connection pin), and not to the pot or star ground. I read that it's supposedly the Right Way in the famous Rane Note. (Unless I brutally misunderstood what I read).

I think the Rane note is spot on and you've done that part right.

However, since it sounds like the hum travels from left to right channel depending on pot position (though much less audible in the left channel due to high attenuation when pot is at low volume) something else seems amiss.

Have you tried either @Laowei's suggestion (amps directly to star ground rather than to PSUs) or first tying PSU grounds together and then use a single connection rather than two from PSU to star ground?

Also, I would probably go for pot to either floating as per Laowei or to star ground, rather than amp boards.

Edited August 20, 2017 by MLA

[gepardcv]

You guys rock. Disconnecting the pot grounds from everything did the trick. The amp is now dead silent. I'll edit my other post to show the exact grounding scheme I ended up with.

[luvdunhill]

How closely to an opamp’s stated “absolute maximum” voltage ratings would you guys run? The devices in question are OPA211 and LMH6321 are behind a regulated supply. They are rated to 36v so I was thinking of running within 10% of that. With easy to find resistor values in my regulator, that is +/- 17.7v, but I would try and find something a bit lower to hit the 10% goal. Power dissipation isn’t an issue here, just voltage rails.

 

What say the veteran designers of HC?

 

[mypasswordis]

The OPA221 datasheet says supply range up to 36V with absolute max of 40V and LMH6321 says supply range up to 30V with absolute max of 36V. If you're using the same supply for both I probably wouldn't go above 16V rails, just to be safe.

[luvdunhill]

Yeah, same rails for both. I am definitely willing to push it harder than 16v - com’on this is HC!

[luvdunhill]

A another question. Does anyone know of an easy way to bend thin gauge mu-metal? To be effective, does it need to continuously surround, be grounded or any other special layout? I am looking to fashion a case divider to separate the PSU from input section of a phono preamp.

[n_maher]

Might be worth buying a cheap brake if you think you're going to do it even infrequently and have the space.  Otherwise you can turn just about any table into a brake if you're willing to fasten something down to it and do the bending with your hands.  Or do this - http://www.instructables.com/id/Simple-Sheet-Metal-Brake-No-Welding/.

[wink]

Mu metal is very fragile to bends and shocks.

Read the following:- http://www.practicalmachinist.com/vb/general/mu-metal-183445/

[Tinkerer]

I'm looking for a mini panel mount voltmeter so I can have balance and offset readouts all the time on the inside of an electrostatic amp chassis. Needs to run on 15VDC or less. Range to +/- 100 would be nice. I see a ton of them around, but nothing that does negative voltages.

[kevin gilmore]

there should be a number of them available.

like this

http://www.ebay.com/itm/Lascar-V-600-3-1-2-Digit-LCD-Panel-Voltmeter-w-200-mV-DC-Bezel-Mount-/202078808312?epid=665967537&hash=item2f0cd5e8f8:g:XocAAOSwA4dWJmh7

this is better

http://www.mouser.com/ds/2/281/20pc-49051.pdf

make sure you understand that the minus input of the voltmeter is in common with the - power supply input and that you have to build a voltage divider yourself because these typically have a range of +/-1.999

Mikhail failed at this by powering the voltmeter with floating voltage and then using it to measure differential output. Ended up frying every single dvm

Edited October 12, 2017 by kevin gilmore

[wink]

But...  but..... Mikhail Rotenberg is my HERO.....  

Besides, the spelt his surname wrong...  It should be ROTTENBERG... 2 t's......

 

[Blueman2]

On 8/20/2017 at 7:13 AM, gepardcv said:

UPDATED 2017-08-21: Thanks to hints from @Laowei and @MLA as well as suggestions to double-check the entire scheme from @mypasswordis and @Whitigir, I figured out what happened. See edits below.

I have a question about volume pots and hum. On my builds, there's a faint hum which begins when the volume pot reaches 11:30 o'clock, gets louder and tracks to the right channel until 3 o'clock, then disappears at max. This happens with all pots I tried, several TKD601s and an RK50. Any suggestions about what might cause this? I have a feeling I'm making a mistake in grounding, but (1) I tried a few variations cobbled from suggestions on various threads here without success, and (2) wouldn't a bad grounding scheme imply that there should be more consistent noise all the time? Likely related to this, I have found that there is much louder hum across more of the pot range whenever there is no source plugged in.

For example, my Carbon (split PSU) is grounded as follows:

• IEC earth to chassis (through IEC module) and star ground
• Transformer 0 to star ground
• PSU board 1 to star ground
• PSU board 2 to star ground
• Amp board 1 to PSU board 1
• Amp board 2 to PSU board 2
• EDIT: Pot output ground 1 to amp board 1 (this caused problems)
• EDIT: Pot output ground 2 to amp board 2 (this caused problems)

FWIW, I have always made one-box builds so far, and I imagine the transformer contributes to this, but I don't get why only part of the volume pot range manifests the problem. Like I said, the hum is pretty faint with nothing plugged in so doesn't matter much with music playing at reasonable levels, but I'd still like to understand what is going on.

I am having the same problem as gepardcv, but with a slight twist.  My amp is dead silent when volume control is fully off or on.  But starting around 11:00, up to about 4:00, there is a definite hum that sounds like a grounding issue.  So far, I just leave the volume at 100% and set volume level using my input device.  But I would like to somehow solve this last remaining issue with my otherwise wonderful SRX-Plus build.  I am using RCA connectors on my SRX-Plus amp, but the amp itself of course is balanced inputs.  Here is the way I have it connected now:

Here is how I have seen it recommended, but does not show volume control.  This also does not address the issue of my volume control having a common ground/return for L and R, as well as my source having a common ground/return for L and R.   Also, unless I tie I- to Gnd on the terminal block, I get massive hum and weirdness, whereas the recommended method below shows not tieing I- to GND.  .  

Anyone have experience hooking RCA input to a balanced amp using a 3 terminal per channel volume control?  

Edited October 14, 2017 by Blueman2

[Pars]

Your hand drawn diagram above appears to be correct, and does the same thing that the Neutrik RCA->XLR adapters do (tie the - signal to gnd).

For wiring interconnects, etc., many tie the shield to the - side conductor, sometimes at one end only (telescoping ground). This is for noise rejection.

For the pot wiring, if looking at most pots from the front with pins down, the furthest R of the 3 pins per gang is the input, and equates to the signal in your drawing on the left side of the pot. The middle pin is the wiper, and equates to the right side of your drawing. The pin furthest L is the gnd or common pin.

[Blueman2]

Thanks, Pars.  I double checked how the wiring is done on the volume control. I am using a pre-made board to mount the volume control with plug in connectors for Input and Output.  It matches exactly what you describe in terms of pins.  I tried different locations for grounding.  Grounded just as the RCA comes into the case to the case or to the star ground.  Tried only connected to ground at the balanced input connector on the amp (tied to I- pin).  Still the same hum when pot is set mid way in range.  None of my input wires are anywhere near AC or transformers.  What is odd is the the hum entirely goes away when the volume control is at 100%.  I do have the RCA inputs connected to a source (O2 DAC, turned on, but no music playing).  Hum is equal in both channels.    

[Whitigir]

My bad, you have problem with RCA.  It seems the best way to do it is to change sources and cables being used.  Rca is SE, so shield is the same as return signals 

maybe try to cut the shield going into the amp, but keep the return signals for music, and keep the shield on the source end.  Doing this will deflect noises toward the source chassis instead of going into the amp

Edited October 14, 2017 by Whitigir
Corrected

[Blueman2]

So I have decided to build another SRX-Plus amplifier but keep to XLR balanced inputs rather than RCA.  But that brings up new issues with volume control.  Do I need a quad potentiometer?  I am pretty sure I will need to reduce the volume equally on both the + and - signals for each channel, so 4 pots (quad)??

Edited October 22, 2017 by Blueman2

[mwl168]

Yes, you need a quad potentiometer for two channel, balanced inputs.

[Blueman2]

I found this drawing by Rob Squire of AudioTechnology, which shows how to do 2 channel balanced volume with a regular dual gang volume control.  This will attenuate the signal by about 6db, but I think this is acceptable for most sources.  The issue for me would be using unbalanced input by shorting cold to ground.  In that case, since colds are tied together with ground, there will be a linkage between left and right through the 2 10K pot resistors: 20k between left and right when full volume, even less when volume is lowered.  As you near lowest volume, left and right will be almost 100% connected!!  That leakage probably kills this idea, right?  Thoughts?

 

 

This is the wiring recommended by DACT using quad volume control:

Edited October 26, 2017 by Blueman2

[Ulfar4]

Hello everyone, I am going to build a DIY esl headphone but first I need some amplifier. I really do not want something fantastic, just to try out the headphones and then build something better. My inspiration was from John Broskie Tubecad schematic and head-fi forum. My question is will it play? And if not, what to change? Thanks for your help.

[kevin gilmore]

the 2 channel pot to do balanced is the same crap eddie current uses. it can be made to work, but causes all sorts of problems. All 4 wires are with respect to ground so not what you think. Also you need a reverse log pot for it to work right, and no one makes those.

electrostat, dn2540 goes boom at anything over 400v. and its a depletion mode part, so not a great idea. what you are doing is a hev70 copy, look at the original for better ideas. also the opamps driving the mosfets have the feedback wrong.