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PostPosted: January 3rd, 2023, 4:36 pm 
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Roscoe,

Interesting thread...

https://www.diyaudio.com/community/thre ... ns.359388/

David


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PostPosted: January 3rd, 2023, 5:19 pm 
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When dealing with DC coupled solid state op amp circuits, whether IC or discrete, you have to take into account the input bias current. It may be quite low in some iterations, but with the circuit gain it can result in problems. For example, Orchard Audio has an unbalanced to balanced converter that one of the members of PAAG bought to use with his balanced only Starkrimson Ultra amp. I'm including the schematic.
Attachment:
Orchard Audio RCA to XLR.jpg
Orchard Audio RCA to XLR.jpg [ 80.49 KiB | Viewed 670 times ]

He complained about the offset so I measured it under different conditions. Note that the two input amp sections are in parallel and connected as a buffer which means the negative input terminals are directly connected to outputs which are a very low impedance. However, with no input connections, both positive terminals connect to a 47K resistor to ground, a very different impedance. The combined bias current passes through the 47K resistor which results in an input voltage at the positive terminals. With the inputs open, that input voltage resulted in an output voltage at the second stage of 280mv, a disaster since it was going into a DC coupled amp

If you short the input jack giving a low impedance path for the bias current, the offset drops to around 1mv. Using a low output impedance device as a driving source would create no problem. Using a high output impedance device is a different story and would result in significant offset. If you were using caps at the output, a simple solution, you will block any DC offset, but that is not an optimal way to go because you are wasting some of the output capability of the device. It is better to use a servo controlling the input DC voltage to keep the offset low while maintaining the maximum capability of the circuit.

Tom


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PostPosted: January 3rd, 2023, 6:05 pm 
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A linestage inserted between the Dave and power amp can only degrade the sound (unless coloration is a goal).

Why not build DPDT selector switches into your amps; run analog through a line stage, the Dave directly.

Just a thought to maximize digital performance.

Stuart


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PostPosted: January 3rd, 2023, 9:48 pm 
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Stuart Polansky wrote:
A linestage inserted between the Dave and power amp can only degrade the sound (unless coloration is a goal).

Why not build DPDT selector switches into your amps; run analog through a line stage, the Dave directly.

Just a thought to maximize digital performance.

Stuart


Minimizing coloration has been the goal, this seems to be a lower coloration way of achieving control at the preamp than using a tube based preamp or even (possibly) a passive. Been thinking of how to achieve your suggestion at the preamp, a bypass switch for the DAC input that would route the signal around the volume pot and active section. May want to think about a relay based solution for this rather than a physical switch. An extra set of interconnects and a switch in the amp also works, but would have to do that on every amp or build an input module with that function that is plugged into the amp input jack. Of course, I can just unplug my amp interconnects from the preamp and plug them into the DAC when I want to change sources, that is the easiest thing to do.

Right now I am just experimenting, and having fun building a neat little preamp. Am I that bored in retirement????

David


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PostPosted: January 4th, 2023, 12:59 am 
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Location: Parkville, Maryland
David McGown wrote:
Walt,


BTW, what is your impression on the XP-12?

Stock -- I wasn't thrilled with it and it wasn't competitive with my modified and upgraded BAT 3-Ki. When I discovered the Pass had E-caps both at the inputs and outputs I went to work. I could dump the output caps altogether and replaced the input caps with WIMA 10-uf pulse caps paralleled with polycarbonate and Teflon caps. Since it was easy to work on I cleaned the entire signal path primarily with Vishay bulk-film resistors, bypassed all of the power supply caps, and with Wayne's advice increased the gain from 9-db to 15-db by reducing the feedback which benefited the the overall sound quality. Now -- the comparison between the BAT and Pass? It's not a contest -- the Pass came out far beyond any expectations I had and then some. The BAT has been put on an early retirement.


Attachments:
Feedback & output_circuits -- one_channel.JPG
Feedback & output_circuits -- one_channel.JPG [ 437.89 KiB | Viewed 661 times ]
Pass_XP-12_Final_121222.JPG
Pass_XP-12_Final_121222.JPG [ 465.36 KiB | Viewed 661 times ]
Over_view_PC_boar_ bottom.JPG
Over_view_PC_boar_ bottom.JPG [ 420.43 KiB | Viewed 661 times ]

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PostPosted: January 4th, 2023, 7:26 am 
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SoundMods wrote:
David McGown wrote:
Walt,


BTW, what is your impression on the XP-12?

Stock -- I wasn't thrilled with it and it wasn't competitive with my modified and upgraded BAT 3-Ki. When I discovered the Pass had E-caps both at the inputs and outputs I went to work. I could dump the output caps altogether and replaced the input caps with WIMA 10-uf pulse caps paralleled with polycarbonate and Teflon caps. Since it was easy to work on I cleaned the entire signal path primarily with Vishay bulk-film resistors, bypassed all of the power supply caps, and with Wayne's advice increased the gain from 9-db to 15-db by reducing the feedback which benefited the the overall sound quality. Now -- the comparison between the BAT and Pass? It's not a contest -- the Pass came out far beyond any expectations I had and then some. The BAT has been put on an early retirement.


Walt,
Thanks, that is really good to know. It occurs to me that I maybe should have not increased the feedback (reducing gain) on the BA2018 linestage from the start, after all, a reduction from 11.3 dB to 7 dB is not that much anyway, a couple of notches on the attenuators. I will have to revisit that. Meanwhile, I am letting the new "Design by Mr.K" switched attenuators settle in, the NOS 50K ALPS "Black Beauty" I had in there at the start sounded better, I think. I may need to revert back to a conductive plastic pot.

BTW, to answer to both Stuart and Roscoe, going direct from the DAVE (using it as a digital preamp) into my amps sounds best, no contest really.

David


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PostPosted: January 4th, 2023, 12:19 pm 
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David McGown wrote:
SoundMods wrote:
David McGown wrote:
Walt,


". . . the new "Design by Mr.K" switched attenuators settle in, the NOS 50K ALPS "Black Beauty" I had in there at the start sounded better, I think. I may need to revert back to a conductive plastic pot."

David

One lesson learned from Wayne Colburn that has proved itself (Nelson Pass and Wayne argued about this before making a commitment) is the MUSES 72320 IC Volume Control. Granted that would seem counter intuitive to providing good playback quality, but the chip does not seem to be in the way at all. I would say it might even be better that a more conventional shunt-attenuator. Although the inputs to the chip has to be capacitor coupled (I am not sure about that yet -- I keep getting conflicting information with regards to its application) one can use noble film caps in stead of aluminum electrolytics.


Attachments:
MUSES 72320 IC Edited for file size.pdf [1.93 MiB]
Downloaded 23 times
Top_Volume_Control_Boards.JPG
Top_Volume_Control_Boards.JPG [ 503.99 KiB | Viewed 650 times ]

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PostPosted: January 4th, 2023, 1:03 pm 
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It sounds like I went through a somewhat similar path to preamps. I used triode (76, 6SNx, 1626 etc) based preamps for many years. The final variant was a 1626 design and it sounded pretty good. For some reason I tried passives but never could get happy with resistive attenuation. They sound transparent and clean but squash out the emotional content IMHO. Went with autoformer preamp for many years which kept the emotional content at the cost of some transparency. On a lark, I picked up an mint condition Ayre K5-Xe MP. It is a Toshiba JFET, fully balanced, zero feedback design that measures well and is generally thought of as a good sounding preamp. My thinking was that I could use the Ayre as a reference while I tried various DIY or autoformer designs. Going from my balanced, SS output DAC to the balanced SS Ayre caused me less cognitive dissonance than I might have expected. I actually found the Ayre to sound better than my autoformer preamp, it is also dead quiet and has a low gain structure (4db) that works well in my system. So, don't feel bad about using a solid state preamp in your system. SS DAC > SS preamp > SET amp works for me. PS I still don't like 95% of the SS amps I have heard for some reason.


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PostPosted: January 4th, 2023, 11:26 pm 
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Over the holiday break I built a pair of Williamson monoblocks for a friend in Florida who's been reaching the limits of his SE 300B integrated amp. But he had no preamp for the Williamsons. On a lark I ordered a Schiit Freya S, thinking that worst case the passive mode wouldn't get in the way. Much to my surprise, after a few days of break-in I found it to be a very enjoyable piece--so much so that I ordered one for myself. I'm interested to hear what a week or so of playing does to it, but I have to say that for $600 it's not bad at all.


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PostPosted: January 5th, 2023, 10:33 am 
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Yea, I had a Freya+ and it was pretty decent sounding. The passive path sounded like a typical passive, the SS path sounded like a good SS preamp and the tube pathway sounded like a decent 6SN preamp (although it was quieter than many). I returned it because it ran so darn hot, I was concerned about 85C cap longevity. If I had kept it, I would have modded the chassis to allow more airflow.



Grover Gardner wrote:
Over the holiday break I built a pair of Williamson monoblocks for a friend in Florida who's been reaching the limits of his SE 300B integrated amp. But he had no preamp for the Williamsons. On a lark I ordered a Schiit Freya S, thinking that worst case the passive mode wouldn't get in the way. Much to my surprise, after a few days of break-in I found it to be a very enjoyable piece--so much so that I ordered one for myself. I'm interested to hear what a week or so of playing does to it, but I have to say that for $600 it's not bad at all.


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