balanced preamp 2025

[kevin gilmore]

finally. higher signal to noise ratio, more than 110db.

takes signals up to 20 volts peak to peak unbalanced, 40v peak to peak balanced. no input attenuator that adds noise for those stupidly high voltage dacs.

( requires 30v power supplies)

(volume control in the middle)

thd about .005%

no feedback.

work in progress

4 layer board.

 

 

 

preamp2025f.PDF

Edited Thursday at 06:15 PM by kevin gilmore

[spritzer]

I love it!!   

[Kerry]

Nice!!!

[15lyl]

So complicated !

[15lyl]

Hope Kevin will explain the circuit detail.

[kevin gilmore]

its not obvious what this is?

first stage is the input section from the uberamp 2.  current mode voltage gain. (not that you actually need voltage gain)

differential input, differential output.  then balanced step attenuator. constant input impedance load.

output stage is the same input section with the darlington input removed because its not needed and some of the cascoding removed because its also not needed, tied to a voltage buffer.

many of the expensive preamps out there convert everything to single ended internally to save on parts and board space. this is balanced from input to output. and 6db or more less noise.

many features of preamps of the past include things that are no longer useful.  so they are removed.  like tape out and second zone. seriously i have never heard of anyone actually using second zone. and except for the really expensive ages old professional gear, there are no tape machines with balanced inputs. maybe the new revox has balanced inputs. 

so consider it as a balanced buffered volume knob.  

turns out the signal to noise boost really is audible.  krell fpb600 vs mc611.  more than 12db signal to noise boost on the mc611 partially because of the low input impedance and rail to rail input buffer.

 

[spritzer]

Nothing more fun than peeking inside a "balanced" preamp to find a dual gang volume control...  🙄 

[dsavitsk]

1 hour ago, spritzer said:

Nothing more fun than peeking inside a "balanced" preamp to find a dual gang volume control...  🙄 

[kevin gilmore]

the idiot at eddie current was big on that circuit. and did that on his electra.  and then tried to get too much gain out of it. and the tubes went into saturation and then you have to hand pick the tubes etc.  biggest problem with that one is the 100k pot starts off with 2nv of noise all by itself.

[spritzer]

Yeah, never seen anybody except the hack Craig actually use that one. 

[kevin gilmore]

actually the bryston uses that scheme.

 

[spritzer]

Another F-tier company.   

[PretentiousFood]

I may be a hack, but I switched from using 4-gang pots to shunting stereo pots because I think they work better.

A big part of the reason why I would build a balanced circuit is to benefit from its CMRR (and even order distortion cancellation by proxy). I measured the matching between gangs on a 20k RK27; it's better than the specified 20% tolerange, but it's not great. Here are some measurements at arbitrary rotations.

Gang A	Gang B	Matching
51.4	51.6	0.4%
255.7	252.8	-1.1%
623	624	0.2%
1299	1300	0.1%
3042	3089	1.5%
4230	4368	3.2%
8470	9080	6.7%
14680	15420	4.8%
19100	19930	4.2%
19900	20380	2.4%

The matching is directly proprotional to the CMRR. So I took a measurement of CMRR using the following circuit.

The idea is that this simulates the balanced input of an amplifier and will tell us how balanced things actually are.

Not very, it seems. Each trace is a different rotation angle on the pot, with the -115dB one being with the signal fully shunted. (There is some resitual resistance with the pot at 0.) -35dB of CMRR will significantly degrade an amp's performance.

I repeated the measurements with a shunting pot configuration, using Yageo 1% 2k resistors and the same 20k pot. 

Although the AP input capacitance degrades CMRR at high frequencies, even the worst 1kHz measurement is about 12dB better than the best measurement with the series pot. The low frequency degradation is most likely caused by the AP's 1/f noise-- we're measuring microvolts at this point.

If I wanted to further improve performance, I'd use resistors with better matching. The Vishay ORN arrays are available in 0.01% and would probably be my pick. LT5400 isn't bad either. 

https://www.mouser.ca/ProductDetail/71-ORNTA1001ZUF

I don't know that this is necessarily relevant to the design in the first post-- looks like it doesn't use a pot at all-- but the circuit has its place.  

[sbelyo]

It looks neat, I like it.  Still gotta build the Iron Pre kit from diyaudio but what's another preamp.  I think I have the transistors too

[simmconn]

4 hours ago, PretentiousFood said:

I may be a hack, but I switched from using 4-gang pots to shunting stereo pots because I think they work better.

A big part of the reason why I would build a balanced circuit is to benefit from its CMRR (and even order distortion cancellation by proxy). I measured the matching between gangs on a 20k RK27; it's better than the specified 20% tolerange, but it's not great. Here are some measurements at arbitrary rotations.

Gang A	Gang B	Matching
51.4	51.6	0.4%
255.7	252.8	-1.1%
623	624	0.2%
1299	1300	0.1%
3042	3089	1.5%
4230	4368	3.2%
8470	9080	6.7%
14680	15420	4.8%
19100	19930	4.2%
19900	20380	2.4%

The matching is directly proprotional to the CMRR. So I took a measurement of CMRR using the following circuit.

The idea is that this simulates the balanced input of an amplifier and will tell us how balanced things actually are.

Not very, it seems. Each trace is a different rotation angle on the pot, with the -115dB one being with the signal fully shunted. (There is some resitual resistance with the pot at 0.) -35dB of CMRR will significantly degrade an amp's performance.

I repeated the measurements with a shunting pot configuration, using Yageo 1% 2k resistors and the same 20k pot. 

Although the AP input capacitance degrades CMRR at high frequencies, even the worst 1kHz measurement is about 12dB better than the best measurement with the series pot. The low frequency degradation is most likely caused by the AP's 1/f noise-- we're measuring microvolts at this point.

If I wanted to further improve performance, I'd use resistors with better matching. The Vishay ORN arrays are available in 0.01% and would probably be my pick. LT5400 isn't bad either. 

https://www.mouser.ca/ProductDetail/71-ORNTA1001ZUF

I don't know that this is necessarily relevant to the design in the first post-- looks like it doesn't use a pot at all-- but the circuit has its place.  

I think you are not measuring CMRR, but rather the common mode to differential mode conversion ratio. With the ‘conventional’ dual pot connection, the common mode signal at the input is attenuated at the same ratio as the differential signal; while with the shunt connection (without the help of a line isolation transformer), the common mode signal is not attenuated at all. Apx has common mode drive, why not use it to evaluate CMRR?

[Shawn]

So complicated. Like the volume mute function.

[PretentiousFood]

9 hours ago, simmconn said:

I think you are not measuring CMRR, but rather the common mode to differential mode conversion ratio.

I think you're right-- thanks for picking up on this. I do still think it matters; if the CM signal is getting converted to DM, the system's CMRR will be limited, no? 

Bill Whitlock of Jensen makes a big kerfuffle about balanced impedances. Here's an AES presentation that you may have seen. It's a bit goofy, but gets the point across.

https://www.aes.org/standards/webinars/AES_Standards_Webinar_SC0505_20210726.pdf

 

9 hours ago, simmconn said:

 Apx has common mode drive, why not use it to evaluate CMRR?

If I'm being honest, I didn't realize there was a CMRR test. 😁 I'll be using this more often.

It can't do sweeps and I don't think it can append results in APx500 v4, but here are some results tested at 1kHz with a 1VRMS signal.

 

 

I tabulated some data at different positions. Looks like there is a "sweet spot" with the conventional pot hookup, whereas the shunt pot does very well once you clear the first few degrees of rotation.

 

Position	Shunt pot CMRR (dB)	Series pot CMRR (dB)
Min	50.800	16.024
	77.772	23.607
	89.748	27.03
	99.328	29.009
	98.410	30.802
	97.917	39.849
	97.656	73.718
	97.362	67.308
	97.228	40.310
	96.271	35.341
	95.442	30.717
	94.715	32.493
	94.031	45.187
Max	93.844	86.054

[simmconn]

9 hours ago, PretentiousFood said:

I think you're right-- thanks for picking up on this. I do still think it matters; if the CM signal is getting converted to DM, the system's CMRR will be limited, no? 

Bill Whitlock of Jensen makes a big kerfuffle about balanced impedances. Here's an AES presentation that you may have seen. It's a bit goofy, but gets the point across.

https://www.aes.org/standards/webinars/AES_Standards_Webinar_SC0505_20210726.pdf

 

If I'm being honest, I didn't realize there was a CMRR test. 😁 I'll be using this more often.

It can't do sweeps and I don't think it can append results in APx500 v4, but here are some results tested at 1kHz with a 1VRMS signal.

 

 

I tabulated some data at different positions. Looks like there is a "sweet spot" with the conventional pot hookup, whereas the shunt pot does very well once you clear the first few degrees of rotation.

 

Position	Shunt pot CMRR (dB)	Series pot CMRR (dB)
Min	50.800	16.024
	77.772	23.607
	89.748	27.03
	99.328	29.009
	98.410	30.802
	97.917	39.849
	97.656	73.718
	97.362	67.308
	97.228	40.310
	96.271	35.341
	95.442	30.717
	94.715	32.493
	94.031	45.187
Max	93.844	86.054

I’m surprised to see the series pot only gets 16dB CMRR at min position. The attenuation at that position would have been much higher than 16dB already, for both common mode and differential mode signals. Did you use the same schematic in your earlier post to connect the series pot when using the APx CMRR measurement? I think the source XLR pin1 should be connected to input XLR pin 1, and series pots’ pin 1,3 should be between the source XLR pin 1 and 2/3 respectively, like how you would connect in a real system.

[kevin gilmore]

board file and schematic

 

preamp2025f.PDF preamp2025f - CADCAM.ZIP