Investigating Balanced Tube Preamp

Hello all,
am new to the idea of using a preamp, always thought I could get away with a volume control on the source and directly go into the power amp. However, lately I dabbled using the preamp section of a Yamaha AV1 receiver between source and power amp (ARC VS115) and it has a bit more "foundation" with it. SO now I am infected with the idea of getting a balanced tube preamp and really intrigued by this chinese clone
Hi End AudioResearch ARC Tube Preamplifier Balanced/RCA IN&OUT,12AU7/ECC83+ECC88/6DJ8 Preamp DIY Kit|Amplifier| - AliExpress
There are many questions just arising from the text (it can be found on ebay as well, but the pictures on this ad are better)

Assumptions:
1) the coupling transformers are to connect unbalanced inputs and process them as balanced
2) the rectification and voltage regulation is included (board and part-wise, except the EZ90 tube)
3) hard time finding a transformer fitting the requirement 280V (two sides), this one is quite close 80W pure copper R-core transformer for tube preamp / phono 280V-0-280V +6.3V *2 | eBay
would one get away in connecting the 2nd 6.3V circuit in series to the first one to tap of 2A @ 6.3V and for the series connection 12.6V at (2A, instead of 3A)?
4)not familiar with preamp topology...no need for an output transformer???
5) a quick search would have the total cost for board, transformer, tubes at 335 US, worth it or better get something what looks like the finished (copy)?
12AU7 ECC88 Valve Tube Bile Preamp Pre amp American ARC Circuit Tube HiFi Balanced Bile Preamp Chassis shell/finished machine|Amplifier| - AliExpress
for roughly twice the price
6) I am really interested in building a kit this time (it has been a long time since I did something in the amplification world)

What are your thoughts? Have you build one of these clones yourself?

Cheers
brxl
 
I bought this kit some time ago. Just getting into it to start the build. For power transformers i wanted toroidal to keep electrical noise down. Had to get two separate ones. I am currently located in the Uk but will be moving back to Canada at some point so also needed duel primary voltages which was a bit tricky to find.

For the moment i am only interested in balanced inputs and outputs so didn;t bother getting the 600:600 transformers.

Just going over the boards and components it looks pretty good. Would upgrade the coupling caps and tube sockets. Also will upgrade the in/out connectors to gold plated and signal relays to better units.

Will post what else i find.
 
Hi,

Took a quick look at the schematic of the arc lss22 (that's what you mean i think?). Might i suggest an rtp3 or rtp5 from Allen wright. The rtp5 is not that difficult to diy.

Schematics

Balanced is nice but only interesting if the rest of the setup is balanced as well. The power amp at least. I prefer that to SE.

Regards Joris
 
Hi Jazz,

it doesn't need to be a specific preamp, but source and power amp are indeed balanced so thank you for providing that link/inspiration.

I would very much prefer a complete kit at this time, although I admit the circuit looks easy enough to do point to point, especially only using the line stage (as I need no phono right now).

I count 4 tube stages, assuming 2 in each ECC88 that is 2 ECC88s? Any suggestions on a power supply with 400V, 12.6V heating and the 24V? Kit please. If I read a capacitor 25/25 does it mean 25uF at 25V? I am confused by R2, does anyone know the value?

cheers
Brxl
 
Hi,

For the rtp5 hv is 200V. Shunt regulation is highly recommended by mr Wright. I'd suggest a sshv. That needs about 50-100V for regulation so you want 250-300V input for the shunt.

I used a Amplimo/Antek transformer with about 250-0-250v for HV, two times 6,3V heater winding and another winding for the -24V. Don't remember which one.

12V came from the small trannie i use for the power switch relais but you could use a voltage trippler from the heater winding and a lm7812 for H.

Joris

edit: Yes two ecc88 per channel, one for the first stage and one for the second, ecc99 is nice as well. 25uF/25V is correct
 
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Power supply built

Last week finished up the power supply board and powered up. Blew the first fuse (1A fast blow). Switched to a 3A slow blow and all ok. Probably just a bit low rating on the first one.
Output voltages all very stable, although no loads yet. It's a clever design with very stable voltage references (series diodes).
Some of the capacitor values supplied are not quite the same as the schematics. They suggest spacing the components off the PCB a bit for heat dissipation which seems like a good idea as tube amps run hot.
I'm using a Toroidy TSTA 220/001 power transformer. Might not keep it for the final build.
Now on to the amplifier board.
 

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Very appetizing pictures. The boards look like a good quality. What is the clever part, the defined voltage drop over diodes?
Thanks for the input on the transformer.

Keep up the good work, I still like the kit idea for easier sourcing than part by part,

cheers

Brxl
 
The diodes provide a very stable and predictable voltage reference for the valve plate b+ supplies. They just put enough in series to get the required reference v.
The PCB's seem to be of good quality, at least to my eye.
I do see some areas for improvement on the audio side, mostly with component quality. Will eventually get better coupling caps, switching relays and audio connectors at least.
The pcb's have the valves mounted on the same side as the rest of the components. It's clearly designed to be mounted inside a case, that is, with the valves inside and unseen. If you like the look of the valves being visible then different mounting and connection will be required.

Will post when i finish and fire up the audio board.

BTW, i'm more of a builder than a designer, so some of my assumptions on the functions might not be bang on.

Cheers
 
Audio board build and test.

Finished the audio board, bench tested and listened.

It tested very well with essentially flat frequency response from 15hz to 24khz. Didn’t bother going any further. Waveform very clean. Also did well with square waves with no ringing or overshoot/undershoot. Could not get it to clip or distort at any volume.

Listening test: Very quiet and clean. Fast. Maybe not as “tube” sounding as some might like. Drives my Class D monoblocks very well.

I hooked up the in and out connectors directly. Still need to build the input/output board.

Really happy with the results. Definitely a keeper.

Attached are shots from my (admittedly) crappy scope.

What's left to do.

Power supply.
Nothing

Audio board:
Upgrade coupling capacitors.
Upgrade volume control
Upgrade gain pots

Input output board:
Buy better signal switching relays
Buy better audio jacks
Build

Overall
Buy case and complete assembly.
 

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For the test build I picked up a Toroidy TSTA 220/001. It has 2 primaries so I can use both in the UK (current location ) or in Canada where i will eventually return( I'm Canadian). Also, I like toroidals for their low EMF. I wanted a very good, quiet preamp.

The amp wants these inputs:
AC/6.5V/2A (±3%) > Transformer Secondary 3
AC/280V-0-280V/100MA (±5%) : Transformer Secondary 1
AC/14V/3A (±10%): I put both Transformer Secondary 2's in series for about 13v.

The amp seems happy with all this. Not sure if i will keep this transformer for the final build as it is physically a bit big.Just starting to look at cases and final layouts.

All tubes and better sockets from Watford valves as I am located in London UK. Great place to do business.

6X4WA-GENERAL ELECTRIC (General Electric)
E88CC/6922-JJ TESLA (JJ Tesla) x2
ECC82/12AU7-JJ TESLA (JJ Tesla) x2
9 Pin B9A Belton Micalex Valve Socket PC Mount VT9-PT (Valves Sockets) x5

Good for testing purposes. I'm sure there will be tube rolling in my future.
 
Hi guys,
I bought this "Ali-ARC" kit before few days - is it possible to find somewhere a schematics for this pre-amp?
I try to redraw it by hand from PCB, but it is not easy as PCB seems to be 4 layer board.
At discussion above was mentioned ARC model LS22, but it seems to me rather as lamp-stage of LS16 (not LS16mkII).
here" https://audio-circuit.dk/wp-content.../audioresearch/AudioResearch-LS16-pre-sch.pdf

Thx, Jiri
 
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Hello all,
am new to the idea of using a preamp, always thought I could get away with a volume control on the source and directly go into the power amp. However, lately I dabbled using the preamp section of a Yamaha AV1 receiver between source and power amp (ARC VS115) and it has a bit more "foundation" with it. SO now I am infected with the idea of getting a balanced tube preamp and really intrigued by this chinese clone
Hi End AudioResearch ARC Tube Preamplifier Balanced/RCA IN&OUT,12AU7/ECC83+ECC88/6DJ8 Preamp DIY Kit|Amplifier| - AliExpress
There are many questions just arising from the text (it can be found on ebay as well, but the pictures on this ad are better)

Assumptions:
1) the coupling transformers are to connect unbalanced inputs and process them as balanced
2) the rectification and voltage regulation is included (board and part-wise, except the EZ90 tube)
3) hard time finding a transformer fitting the requirement 280V (two sides), this one is quite close 80W pure copper R-core transformer for tube preamp / phono 280V-0-280V +6.3V *2 | eBay
would one get away in connecting the 2nd 6.3V circuit in series to the first one to tap of 2A @ 6.3V and for the series connection 12.6V at (2A, instead of 3A)?
4)not familiar with preamp topology...no need for an output transformer???
5) a quick search would have the total cost for board, transformer, tubes at 335 US, worth it or better get something what looks like the finished (copy)?
12AU7 ECC88 Valve Tube Bile Preamp Pre amp American ARC Circuit Tube HiFi Balanced Bile Preamp Chassis shell/finished machine|Amplifier| - AliExpress
for roughly twice the price
6) I am really interested in building a kit this time (it has been a long time since I did something in the amplification world)

What are your thoughts? Have you build one of these clones yourself?

Cheers
brxl

Hello!
See schematics in picture.
 

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PCB&parts for sale

Hi all,
as mentioned earlier, I have bought PCB and parts, version with XLR and coupling trans.

Because I found meantime that circuit differs from ARC16 remarkably and there will be no easy to modify this PCB, I decided to sell this set.
Meantime I used a few (very usual) parts from this kit (24V zener diodes, few resistors, one 7812, two LEDs, etc. - but all "exotic" and special parts for this kit, like pots, power supply trasistors and all capacitors are here; please note this kit when new did not contain tubes and its noval-sokets).

Currently it cost at Ali about 200,- EUR + transport.
so I guess my price let say 50,- EUR (+ transport from CZE - I prefer shipment at EU).

If someone interrested, please send PM.
thx Jiri.
 

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