John Curl's Blowtorch preamplifier part II

[buzzforb]

I have the same problem, they quote yield issues which is perfectly understandible but there is no business argument in only supplying DIY audiophiles for phono circuits.

A bird told me they have OEM's and distributors waiting with massive orders.

 

[john curl]

That may be true, BUT even the 40 year old j175 is out of stock,(DigiKey) yet there are 3 established manufacturers for the part, today. In the past, everybody and his brother made j175's, but no more.

 

[john curl]

This is the problem: buzzforb.
Audio amateurs are the biggest pita's that a manufacturer can have. They want small quantities, and low prices, (samples preferred) and they are VERY FUSSY about what they get.
Let us hope that Linear Systems will finally release the devices IN QUANTITY to distributors, who are set-up for this sort of customer. Today, no.

 

[EUVL]

I presume they will buy a few million USD's worth, whatever the yield ?

🙂


Patrick

 

[buzzforb]

This is the problem: buzzforb.
Audio amateurs are the biggest pita's that a manufacturer can have. They want small quantities, and low prices, (samples preferred) and they are VERY FUSSY about what they get.
Let us hope that Linear Systems will finally release the devices IN QUANTITY to distributors, who are set-up for this sort of customer. Today, no.

Diyer's, fussy and cheap. Cmon🙂

 

[gk7]

... an example of an ALTERNATE topology ...

John, I remember you posted an earlier version of this circuit with
additional mosfets in the output stage. In this version these are removed
and in turn the current through the BJTs increased somewhat.

My question is: were these mosfets removed because the extra current they
provide was not needed for the application (2nd stage of phono preamplifier)
resulting in fewer parts, or were you not satisfied with the mosfets versions performance ?

In other words, if one needs more current, is the "mosfet version" still a
valid approach or are there certain caveats ?

 

[Waly]

That may be true, BUT even the 40 year old j175 is out of stock,(DigiKey)

No

 

[john curl]

Well GOOD! Waly I just did a search, but I'm glad to be wrong.

 

[hitsware]

JFET | Mouser

Search results for "" - Allied Electronics

https://avnetexpress.avnet.com/stor...=p+channel+jfet&topSellers=&categoryLink=true

 

[elektroj]

John, you keep mentioning j175 - j113 combo occasionally.. According to the datasheets:

j175 - Vgs(off) = 3-6 V, IDSS = 7-20 mA
j113 - Vgs(off) = 0.5-3 V, IDSS = 2-(?) mA

Are they really complementary? Or, does one, in order to get more or less complementary pairs, has to select from a large number of devices?

 

[john curl]

So?

 

[john curl]

Actually, the problem is still real, unless you go surface mount.
However, these early devices are close enough to make a successful complementary differential jfet input. Of course, the Toshibas are better, that is why we switched over to them more than 30 years ago.

 

[john curl]

OK, everybody. Let's look more carefully at the data sheets for the 'alternate' jfet devices:

 

[Bonsai]

Here is an example of an ALTERNATE topology that doesn't have pchannel jfets, that is akin to what Scott Wurcer used for his AD797.

I am very concerned about the availability of P ch jfets. Somebody, somewhere is grabbing ALL of them up. Please input here as to your problems and what you have done to find these pch parts.

Nice circuit. Just buffer it and there you have it. No complementaries needed.

 

[john curl]

Or don't buffer it, and it will work pretty well too.

 

[john curl]

I just found a good compromise:

 

[elektroj]

OK, everybody. Let's look more carefully at the data sheets for the 'alternate' jfet devices:

John, which parameters specifically shall we look at "more carefully"?

 

[john curl]

The graphs are really important to fix bias, etc. Look at the non-linear input capacitance and the feedback capacitance. Note ways to improve these factors. etc.

 

[EUVL]

John,

What MOSFETs would you suggest for #38676 ?
I find it also difficult to find true-complementary small signal MOSFETs.


Thx,
Patrick

 

[john curl]

I generally use IRF610 and IRFP510 devices of output followers or cascodes. For common source amplifiers, these devices have to be at least Harris (Fairchild) for the P's. Hitachi devices like the 2SK214, 2SJ76 work OK too.
The 'complementary' argument is sort of weak. It is not that true complementary devices would not be desired, but the lack of true complementary parts usually just means a bit more 2'nd harmonic. It must be remembered that usually the complementary part is working WITH the other part, not just being a parasite, just drawing current (at minimum) or even adding to the load, AND as well having a complementary distortion curve as well.