The ultimate DIY? A Stax SRM-T2!

[Craig Sawyers]

Then there is the fairly new Elma A47 (47 positon) four channel attenuator https://www.elma.com/en/products/rotary-switches/audio-solutions/a47-audio-switch--4-wafers-12070365

But you are back in RK50 territory.

The conclusion is: highest quality four channel attenuators all end up at about the same price, or even beyond RK50!

[starcat]

Why did actually the T2 end up using a 50k pot and not like the other KG designs, BHSE, etc a 10k? I think at 2012 time frame the RK50 was available in 10k so the DACT (which still is). 

Edited July 16, 2021 by starcat

[johnwmclean]

4 minutes ago, Craig Sawyers said:

The conclusion is: highest quality four channel attenuators all end up at about the same price, or even beyond RK50!

Goldpoint V47 is same Elma switch and a lot less money.

 

[mwl168]

4 hours ago, Craig Sawyers said:

Then there is this http://www.tkd-corp.com/products/att/pdf/p65cs-e.pdf which is what Curl used in the Blowtorch. He used a stereo one, not four channel, because Blowtorch was not inherently a balanced preamp.

It is available in four channel, I have no idea of the price, but expect it to be substantial.

Then there is the Seiden 74005, 58 position 4 channel switch. You need to add resistors of course. That switch is Y47,000 direct from Seiden ($430, shipping, taxes etc on top). Depending on the resistor choice you could be back in RK50 territory or beyond.

Although the Seiden price list is 2012, that is the one that Seiden sent me in Feb 2020.

I have not tried either the TKD nor the Seiden, but at least considered the Seiden for another project (pending) that needs a really weird attenuation law.

SEIDEN.pdf 531.84 kB · 4 downloads PRICE_LIST_2012.10.pdf 315.56 kB · 4 downloads

That TKD pot (2-gang) is also used in the Audio Note M7 preamp.
I looked up its price at Michael Percy’s site many years ago. I don’t remember the exact number, it was expensive but I did not choke on it. 

Edited July 15, 2021 by mwl168

[Pars]

Another pot or stepped attenuator I have worked with (on Todd's CFP3) is a 41 step unit from Acoustic Dimension. It seemed pretty nice with good tracking, etc. The do make quad units, and the one in the CFP3 was a 10K, which they don't show on their website.

http://www.acoustic-dimension.com/

 

[starcat]

Had to use tricks 🙂 to hold the 10PPSG washer in place on the K216 as it rotates awfully during tightening.

Lifted the 6DJ8 teflon socket as much as possible as they are lower than the 6CA7. Resistors are all Xicon 273 series and raised 1.5mm from the board.

Edited July 17, 2021 by starcat

[starcat]

Would like to test the J79 and C3381 prior soldering them. Anyone can help? Every other component I have tested on its own prior soldering.

Decided to use the KSA1156 on the heatsinks instead of 1486, despite having them. From what I've read it shouldn't matter at all. Everywhere else using 1486. No other mods whatsoever. Curious to hear what Dr. Gilmore thoughts are regarding this. @kevin gilmore

Thank you, highly appreciated. 

Edited July 17, 2021 by starcat

[starcat]

Any good source for matched 6922 or the European made E88CC with gold pins, like Amperex, Telefunken, Philips, Siemens?

Edited July 17, 2021 by starcat

[Pars]

Good NOS (heh) 6922s are probably rare as hell anymore. When I used to have a tube preamp (Counterpoint SA5.1) 10 years ago, I had a pair of Amperex PQ white label 6922s in it, which I really liked. The EH 6922 gold pins are quite good. If you can manage to find some vintage, you will pay out the nose for them, and possibly get ripped off.

Some seemingly decent recommendations for dealers here: https://www.thegearpage.net/board/index.php?threads/online-tube-dealer-sticky.1197164/

[Kerry]

The JAN Phillips are a nice choice to get started with though I prefer the Amperex. 

https://www.audiotubes.com

Tube depot has a matching service as well. 
 

PS. Not sure about where to buy in Austria

Edited July 18, 2021 by Kerry

[starcat]

Thanks, guys!

Found this here as well https://www.head-fi.org/threads/6922-tube-review-17-top-6922-6n23p-e88cc-cca-7308-e188cc-tubes.761078/

Looks like the Valvo E88CC CCa Herleen white imprint and gold pins is a best buy for around 90/pair here in Europe.

Interesting the list above is headed by a Russian REFLECTOR make with silver shields specifically from 1975, even in front of Telefunken or Siemens. They are talking "solid state fast and precise, but with tube liquidity and warmth". 

Edited July 18, 2021 by starcat

[Rinat]

I've been trying to understand what should I measure to be sure that a transistor is genuine, for example in case of 2SK246-GR. But what if I will replace it with 2SK208?

It is still in production and it is just a SMT version of 2SK246, with lower max power dissipation (100 mW vs 300 mW).

But in the original scheme of active battery power dissipation of the JFET is around 1-2 mW. So... It would be ok to use 2sk208. 

Am I right ? 

Thanks and I hope it would help someone else 🙂 

[JoaMat]

I think you are right.

I’ve used 2SK879 in one T2 battery since several years without any issues. From datasheet 2SK208 and 2SK879 look quite similar.

[Rinat]

I've definitely helped someone, who had immediately bought all 2sk208-GR from digikey 😄. There is only an R-grade on mouser/digikey with very low Idss now ((

Did someone try to buy something at arrow.com ? They have 2sk208-GR at japanese warehouse and FQP8N80C. 

[Craig Sawyers]

I think I know who that was. And it is not someone on this list or associated with it. I mentioned the 2sk208-GR to him by email, and he told me he had previously bought 3,000 of them, before I even mailed him.

By the way, TI have just introduced a low-noise n-channel JFET, the JFE150 (0.8nV/root Hz, and 1/f corner at 10Hz) https://www.ti.com/product/JFE150 . Apparently this is just the first in a series they are planning, with n-channel monolithic duals (apparently imminent), and possible p-channel versions.

They are quite serious about the audio market, with a superb and growing portfolio of audio op-amps.

Unlike On-Semi; don't start me on that soap box....

Edited August 20, 2021 by Craig Sawyers

[Rinat]

A few thoughts about replacing transistors to a contemporary parts: 

1. I thought that stn9360 is just an ersatz for 2sa1486, but it seems that it is even better than the original one. 9360 has mush higher hfe and a little lower capacitance. It means that it is more suitable for the active battery and CCSs 

2. KSA1220A has lower Vbr (160v instead of 200v for 2sk216) but it is ok even in original scheme. Does it mean that 2sk214 is safe to use ? I also afraid to get lower sound quality because of poor linearity of this mosfet (. 2sk216 is much better in this term. So, I am trying to find better substitution 🙂 . I don't like to use obsolete parts. It makes me slightly nervous. Now I am going to use 216/79 but I want to build someday a "fully-contemporary parts" T2 🙂.

3. 2SC3675: FJPF2145 has lower hfe and Vbr (. I have enough 3675s but also want to substitute them in future. Maybe cascoded CCS using JFET + DMOS ? It has much better resistance(look at the photos from the book) than single-bjt CCS.  

Cascoded CCS measured resistance

Single BJT CCS measured resistance

4) 2SK246 -> 2SK208 

5) 2SC3381 ->  HN4C5J . What about matching of halves ? There is no info about max difference in datasheet. 

Also I want to build the amp with replaceable active battery - soldered to the main board in 2 points. I don't trust mechanical connection. 

UPD: It looks like KSA1220A is obsolete now (official status -  "last shipments")

 

[JoaMat]

 I use/have used those CCS. Good for +/-500V.
 

I’ve replaced 2SA79/2SK216 with KSA1220/KSC2690. Looks like KSA1220 is ended, but TTA004/TTC004 should be fine.

[bbest]

Tried to estimate possibility to collect BOMs for T2 on mouser. First struggle was with MOQ=100 of resitors 273 Xicon MF-RC. Second step is long lead time. Some examples:

595-LF353P
512-KSA1156YS
747-IXCP10M90S
512-FQPF8N80C
512-2N3904BU
655-RT424012
512-BC547BTA
512-KSC5026MOS
512-FQPF8N80C
512-2N3904BU
863-1N4007RLG

Baddest times for DIY arrived 😅

[jamesmking]

19 hours ago, bbest said:

Tried to estimate possibility to collect BOMs for T2 on mouser. First struggle was with MOQ=100 of resitors 273 Xicon MF-RC. Second step is long lead time. Some examples:

595-LF353P
512-KSA1156YS
747-IXCP10M90S
512-FQPF8N80C
512-2N3904BU
655-RT424012
512-BC547BTA
512-KSC5026MOS
512-FQPF8N80C
512-2N3904BU
863-1N4007RLG

Baddest times for DIY arrived 😅

100 xicon resistors are barely more than the cost of 10 koa resistors. If you are planning to never build anything else the xicons are a waste, otherwise its good to have spare stock. With 100 resistors of each type you can also hand match the resistors between + and - of a channel and between channels 🙂  For the RT424012 you could use instead the RTE24012 which has the advantage if being sealed - so better if you are going to ultrasonically clean the pcb. IXCP10M90S is an issue - out of stock world wide. BC547BTA can be substituted by BC547B which is in stock at mouser - you just have to bend the legs a little otherwise its the same transistor.

I don't see why any 1n4007 can't be used - its a generic jelly bean component. mouser has 2n3904 available from central semiconductor - again you may need to bend the legs

Edited October 6, 2021 by jamesmking

[bbest]

Yes, you are right. But without IXCP10M90S all work is useless. May be it make sence to buy maximim parts now and then the rest of another. Because when IXCP10M90S will be available, could be absent another items. The question that it could be loose some money on extra shippings.

[jamesmking]

I tend to buy in bulk when something is available (for expensive components: double what I need for a build, for inexpensive components like resistors, diodes, zeners, bc5XX, KSP92/42 transistors etc I buy 100), because:

1. bulk discounts,

2. free shipping if you spend above a certain level,

3. useful to have spares so if you make a mistake in building it initially or in 10 or 20 years time you can still service and repair the amp... 

4. you never know when the there will be more stock.

5. you can sometimes swap parts with other people

6, you never know when ON semiconductor will delete the component or take over the manufacturer (as they did with Toshiba) and then delete all the useful parts

7. component prices only go up especially when they are no longer manufactured

8. you are less reliant on ebay or other doggy sources of possibly fake components.

9, more choices if you need matched pairs, or need high hfe/Beta etc

I keep my inventory in an excel spreadsheet, so I know what I have and what I am getting low on. I also add to the sheet what build(s) each component is used in. Which helps with making decisions on quantities and buying priorities. 

for example I have 24x 10m90s because they are used in the golden reference hv psu - which I use in every build I can and are used in the  modern t2 psu, T2 amp, and blue hawaii...

Edited October 6, 2021 by jamesmking

[bbest]

Good approach 👍 I also trying to buy a litte more, but not too much. From my practise all projects completely unique. So parts are not shareable. I built several general amplifiers, balanced beta22, some DACs and so on. As for repairing: mine projects are very cool and I am trying to build them very reliable, so don't need to repair them. If realy something will broke, I will better build new more modern and more performance gardget. This is from my practise of 8 years in DIY. So in conclusion: if extra parts doesn't costs too much - then buy them, otherwise exact count or +1.

But anyway: it is usefull to buy 10m90s and fill excel with extra inventory - good idea.

[Rinat]

A question for anyone who calculated the amplifier - what is the point in using high-voltage transistors in current mirror in the active battery ?

The current through the each transistor is ~0.05 mA, Ur41 and Ur40 ~ 5 mV. It means that Uce of the right part of 2sc3381 should not be more than ~5v (It is the max Ube of 2sc3675). And the same is for the left part: Ube is 5V max -> Uce max = 5v. 

So - what is the point ? There are nice double transistors with lower maximum Uce, with the same capacitance, with higher hfe etc.... 

[JoaMat]

18 hours ago, Rinat said:

There are nice double transistors with lower maximum Uce, with the same capacitance, with higher hfe etc.... 

Which ones do you have in mind?

[Rinat]

4 hours ago, JoaMat said:

Which ones do you have in mind?

These ones for example: 

https://www.mouser.com/datasheet/2/308/1/MBT6429DW1T1_D-2315127.pdf

https://www.onsemi.com/pdf/datasheet/bc846bdw1t1-d.pdf

https://www.analog.com/media/en/technical-documentation/data-sheets/MAT01.pdf

https://toshiba.semicon-storage.com/info/docget.jsp?did=19156&prodName=HN1C03FU

https://www.digikey.com/en/products/detail/linear-integrated-systems-inc/IT120-PDIP-8L/13688035

https://fscdn.rohm.com/en/products/databook/datasheet/discrete/transistor/bipolar/emx26.pdf

 

I would try the first one or MAT01AH if I were sure that I don't need > 80v of Vce at the battery ( 

And I am not sure that we really need here low output capacitance. If no then there are another nice options. 

But I don't think that using separate single transistors is a good idea for current mirror due to temperature drift. I prefer one crystal or at least one package. 

Edited October 25, 2021 by Rinat