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 Post subject: Re: WTB: Tube Preamp
PostPosted: October 27th, 2021, 3:52 pm 
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Joined: July 17th, 2016, 6:24 am
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Roscoe, Can you move this thread to Projects forum and change the title, pl.


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 Post subject: Re: WTB: Tube Preamp
PostPosted: October 27th, 2021, 7:12 pm 
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David McGown wrote:
A single board Octal Aikido w/Power Supply and parts and no tubes is $200 from TubeCAD. Just putting into perspective. You have to bring your own chassis, source selector, and volume control. And the board is in stock!

David


Thanks. Ordered this kit from Glassware. It came to $330 including power transformer and Hammond chassis.
Any recommendation of 6SN7 tubes?

One of the issues I am having now is the Holo Spring DAC outputs DSD 6dB lower than PCM. Would be nice to have a remote control with volume display on the preamp. Tubes4Hifi has one for $300. But right now, I am ordering a cheap chines pot with remote and display for $33. If I like the amp, upgrade it later.
https://www.ebay.com/itm/165008936285?h ... Swk4dhEMg1

BTW, I will also build the low-mu 6V6/12B4 preamp. For that, I would assume I might have to spend about $150-$200.


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 Post subject: Re: WTB: Tube Preamp
PostPosted: October 27th, 2021, 8:26 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 487
Cogito wrote:
David McGown wrote:
A single board Octal Aikido w/Power Supply and parts and no tubes is $200 from TubeCAD. Just putting into perspective. You have to bring your own chassis, source selector, and volume control. And the board is in stock!

David


Thanks. Ordered this kit from Glassware. It came to $330 including power transformer and Hammond chassis.
Any recommendation of 6SN7 tubes?

One of the issues I am having now is the Holo Spring DAC outputs DSD 6dB lower than PCM. Would be nice to have a remote control with volume display on the preamp. Tubes4Hifi has one for $300. But right now, I am ordering a cheap chines pot with remote and display for $33. If I like the amp, upgrade it later.
https://www.ebay.com/itm/165008936285?h ... Swk4dhEMg1

BTW, I will also build the low-mu 6V6/12B4 preamp. For that, I would assume I might have to spend about $150-$200.


The Aikido circuit has a *lot* of gain. I find that left and right volume pots linked to a master help fine-tune the gain to match your amp. The Williamsons like a lot of gain so the Aikido works very well for me. I use two Gold Points for the "balance" and a ladder Khozmo for the master. Just something to consider down the line, you might want to leave room for the extra left & right pots.

We can go round and round about 6SN7's but my go-to is the GE 6SN7GTA or GTB. Affordable, consistent from sample to sample and with an open, full, even-handed sound. I suggested them to David for his Williamsons and I think he liked them a lot.


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PostPosted: October 27th, 2021, 9:13 pm 
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Quote:
The Aikido circuit has a *lot* of gain. I find that left and right volume pots linked to a master help fine-tune the gain to match your amp. The Williamsons like a lot of gain so the Aikido works very well for me. I use two Gold Points for the "balance" and a ladder Khozmo for the master. Just something to consider down the line, you might want to leave room for the extra left & right pots.


Although I don't need "lot" of gain, its not a deal breaker. My criteria for the preamp is low distortion, low noise and lots of micro details. That is why I was looking for zero-feedback designs. Aikido seems to fit that bill along with low-mu preamps which David suggested.

I understand the left and right channels might not be balanced. I am using a 5-way active system. In two amps, there is a difference of about 1-1.5dB between the L and R channels.

After the preamp project is done, I am going to start building monoblock amps starting with EL84 based SETs for the top two channels. One from 1400Hz to 8000Hz and other > 8000Hz. Planning on housing both amps in a single chassis powered by a single power transformer. 1st order crossovers will be part of the amps circuit. Planning on implementing an lpad also in the circuit for balance adjustment.


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PostPosted: October 27th, 2021, 9:19 pm 
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Joined: June 4th, 2013, 2:39 pm
Posts: 487
Cogito wrote:
Quote:
The Aikido circuit has a *lot* of gain. I find that left and right volume pots linked to a master help fine-tune the gain to match your amp. The Williamsons like a lot of gain so the Aikido works very well for me. I use two Gold Points for the "balance" and a ladder Khozmo for the master. Just something to consider down the line, you might want to leave room for the extra left & right pots.


Although I don't need "lot" of gain, its not a deal breaker. My criteria for the preamp is low distortion, low noise and lots of micro details. That is why I was looking for zero-feedback designs. Aikido seems to fit that bill along with low-mu preamps which David suggested.

I understand the left and right channels might not be balanced. I am using a 5-way active system. In two amps, there is a difference of about 1-1.5dB between the L and R channels.

After the preamp project is done, I am going to start building monoblock amps starting with EL84 based SETs for the top two channels. One from 1400Hz to 8000Hz and other > 8000Hz. Planning on housing both amps in a single chassis powered by a single power transformer. 1st order crossovers will be part of the amps circuit. Planning on implementing an lpad also in the circuit for balance adjustment.


Sounds good! I like the overall plan. Another thing I like about the Aikido is that it is *dead* quite thanks to the PSU cancellation properties. And the lack of feedback makes for a very natural tone. I'll be interested to see what you think of it.


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PostPosted: October 28th, 2021, 4:19 pm 
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this is the first version of PS schematic for the low-mu preamp. PSUD2 is very limited in functionality. The choice of caps and inductors is strictly based on what I have available.

Todo: Add common-mode and RF noise filtration.

Attachment:
lomu_psu1.JPG
lomu_psu1.JPG [ 153.71 KiB | Viewed 6401 times ]


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PostPosted: October 28th, 2021, 4:44 pm 
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Joined: February 28th, 2013, 1:19 pm
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Shashi,

PSUD2 is much more capable than you give it credit. You need to feed the correct information for equivalent DC resistance of the components, and not use the default values. You also need to model the load correctly, you can use a constant current representing the operating current of the circuit, or use a resistive load as you are using that is based on the B+ voltage divided by the operating current. The actual amount of ripple you are seeing is miniscule, due to the capacitor sizes and inductors used. It is good to see the stepload response to see how much ringing you might have with the two chokes in the Pi filters, these can interact. Sometimes it is good to use two different values for chokes, or even add some resistance to the choke to dampen ringing. For a Class A circuit, it probably does not matter that much, but it is something to look at.

Yes, PSUD2 is NOT going to address common mode or RF noise filtration since this is outside the scope of the application, which is to help model and design a stable and well filtered power supply based on 60Hz/50Hz AC input. If you use the correct input information, including line voltage, component resistance, loads, etc., the modeled results are very accurate. PSUD2 is a great tool for power supply optimization.

David


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PostPosted: October 28th, 2021, 4:57 pm 
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Joined: February 28th, 2013, 1:19 pm
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Shashi,

Also use the rectifiers you intend on using where possible, since it will model the drop across them as well as give you an indication of whether you are exceeding their current capability or PIV. Also, I find that using a 1000mS window is too wide and you cannot see what is happening for all the 120Hz ripple, I like to use a 200mS window. The only case I would use a longer window is if I wanted to see turn on behavior to see if the voltage spikes up excessively and how long does it take for the power supply to charge up.

It is a very useful tool.

David


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PostPosted: October 28th, 2021, 5:30 pm 
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Joined: January 14th, 2015, 11:15 pm
Posts: 499
The following is a good primer for using PSUD.

https://www.dhtrob.com/overige/pdf/dhtrob_psu.pdf

In the meantime, this is what your initial version looks like.

Image

Changing the load resistor R1 to a constant current of 30 ma (15 ma per channel) and setting a stepped load. The stepped load facility is more useful in higher current applications.

Image

Your chokes are too large as well as your input capacitors. Using a $20 Hammond 155H ( 5H @ 50 ma) and just a 10 uf input cap, you get the following if you use 30 ma current (15 ma per channel).

Image

And here is your ripple which should be fine.

Image

PSUD is a wonderful tool.

ray


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PostPosted: October 28th, 2021, 7:05 pm 
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Joined: February 28th, 2013, 1:19 pm
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Ray,

That was a great little demonstration for Shashi on using PSUD2. Since this is for preamp use, I would tend to add another RC after the last, maybe accept a 5 to 10V drop into a large capacitor but get better filtering. Or use the current filter as input into a regulator.

David


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