audio research LS25 preamp

[analog_sa]

You are nuts 🙂


These inductors decouple the electrolytic caps at high frequencies where the bypasses take over. ARC even have a patent on this. One would wonder why diyers have never tried it (myself included).

 

[Bill Fuss]

You're right

I didn't look close enough, must be a part of the sound of these units. Something makes these units special, synergy of the various sections I suppose. After hearing one I wouldn't have guessed the audio circuitry was this simple. Worthy of a Nelson Pass KISS design.

Bill

 

[johnmarkp]

Analog_SA, Do you have the patent number for the inductor resister network. I could make interesting reading.

 

[johnmarkp]

I want to bump the request for help finding Q4 6765 in the power supply section. I think I have everything else figured out on this.

John

 

[analog_sa]

Sorry, no patent number. If you think how many people complain that bypassed electolytics don't sound natural and that bypassing can lead to resonances it makes very good sense. It looks like ARC mostly use the same resistor/inductor values irrespective of capacitance so it's probably not very hard science.

Regarding the mosfet i think 6765 is just and ARC stock code. No reason why an IRF830 won't work perfectly.

 

[johnmarkp]

I have a couple of questions for people who have built this. Did you use a pair of jfets for Q1 and Q2 or just one? AR uses are pair. Also did you build the power supply as shown on the schematics?

 

[analog_sa]

This will be important if you are using the preamp in balanced mode. Either match 2SK170 for Q1/Q2 or use a 2SK389.

 

[johnmarkp]

I think I may not have asked the question right. Each side of the balanced channel has 2 jfets. Take a look at the pictures at www.arcdb.ws.

John

 

[Bill Fuss]

I only see 3 per ch on the schematic, what pic are you referring to.

Bill

 

[johnmarkp]

Go to the arcdb web site and look at the picture of the 25mkII. You can see that there are two jfets at each location Q1 and Q2. You can also see it here... http://he-japu.blogspot.com/2007_01_01_archive.html. It also looks like the R7 and R8 resistors are marked wrong. I think they are actually 1K not 150ohm.

 

[analog_sa]

Your link doesn't work but you appear to be right. As the original fets are unknown is this actually important?

 

[johnmarkp]

Using one or two does not really matter a long as the circuit works and sounds good.

 

[johnmarkp]

I am getting ready to test a prototype for this preamp. I am planning on using the 2SK170 for the input JFET. However I am not sure what the spec for the JFET should be. I have read Borbely's papers and I can not figure out how to determine the proper Idss or how to change the resistors to match the JFET. Can anyone tell me the Idss that matches the resistors used? I will be using 4 2SK170 in two pairs as is done by ARC.

Thanks, math has always been my Achilles heal

 

[SY]

Spelling, too.😀

It's not really a math question. If you use the BL series and match the two input FETs, the adjustment of the tail current will take care of things for you. Reality is, FETs show a HUGE Idss spread from unit to unit, so good designs tend to be rather robust as regards absolute values.

 

[Jaap]

is 2sk30 usable

in this circuit ?

 

[johnmarkp]

Thanks Sy. I was always the first person out of the spelling contest as well. Maybe my wife is right that I have no skills at all.

 

[analog_sa]

Maybe my wife is right that I have no skills at all.

No skills? You have obviously been, at least at some stage, "marriage material"

 

[johnmarkp]

Analog_SA, someday your wit will cause you grief 😉 If it has not done so already.

I fired up my prototype The voltage between R16 and R14 is 55 volts not 70. I can not get over +5 volts on the drain from the 2SK170BL. There are a couple of changes from the schematic that I did to reflect the values shown in pictures of the LS25MkII. Input R7 and R8 are 1k. R16 and R17 are two times 16.9K so ~8.5k instead of the 13.3k in the schematic. I used OK parts and it seems to sound pretty good in my test rig. I am not sure if I should care about the two voltage measurements being off, since this appears to be the way AR made it? Any inputs on this are appreciated.

 

[johnmarkp]

MK1 question

In the LS 25mkI they use a mosfet to feed power to the first set of tubes. It is labeled as "1018". Anyone have an idea what would work there?

Thanks

 

[analog_sa]

Looks like you need something with Idss of more than 7mA. I can't see a reason why a K170BL won't work.

Btw, +5v on the drain is simply not enough if that's what you still have.