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A couple of parts questions

A couple of parts questions

2018-07-06 by Mike Burnham

Hi there

For the Serge Noise circuit it specifies the noise transistor as 2n/PN4250 but these are stupidly expensive nowadays

Is it possible to use BC557? Are they noisy enough? BC557C probably best?


The VCA specifies a matched transistor package

Would matched 2N3904 be OK? Or do they have to be super matched?


Thanks

Re: A couple of parts questions

2018-07-06 by Ken Stone

You may have to try a lot of different transistors to get one that is noisy enough. I selected the 4250 because they were all noisy to the point of some of them being too noisy.
For transistors, the better the matching, the better they work - less bleed through. Try them and see if you are happy with the result.

Ken
Show quoted textHide quoted text
On Fri, Jul 6, 2018 at 9:04 PM, Mike Burnham mikejburnham@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:

Hi there

For the Serge Noise circuit it specifies the noise transistor as 2n/PN4250 but these are stupidly expensive nowadays

Is it possible to use BC557? Are they noisy enough? BC557C probably best?


The VCA specifies a matched transistor package

Would matched 2N3904 be OK? Or do they have to be super matched?


Thanks




--
Ken Stone otherunicorn@...
Modular Synth PCBs for sale <http://www.cgs.synth.net/>

Re: A couple of parts questions

2018-07-06 by Boris Klompus

I'm not sure about the latter question - I assume the 2 matched transistors would be fine - but mouser stocks PN4250A. it's nearly identical according to the datasheet, just has higher capacity for collector-base voltage, and collector emitter voltage. The price is $0.41/ea: https://www.mouser.com/ProductDetail/Central-Semiconductor/PN4250A?qs=sGAEpiMZZMshyDBzk1%2fWi8oN7VHZ91OktECUlj3XHEg%3d

Here are the datasheet for comparison-



‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On July 6, 2018 7:04 AM, Mike Burnham mikejburnham@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:



Hi there

For the Serge Noise circuit it specifies the noise transistor as 2n/PN4250 but these are stupidly expensive nowadays

Is it possible to use BC557? Are they noisy enough? BC557C probably best?


The VCA specifies a matched transistor package

Would matched 2N3904 be OK? Or do they have to be super matched?


Thanks




Re: A couple of parts questions

2018-07-06 by JAMIE logan

using matched pair . 1/ there matched for you. 2/ because they are on same die the first thermal error is cancelled. which is perfect for vca.

3 / Erica synths sell the https://www.ericasynths.lv/shop/diy/diy-accessories/as394-matched-transistors/ for cheap.

time is money and matching transistors takes time.

the transistors ken chose for noise are good the other good one is a 2n5172 but pin out is different.

Show quoted textHide quoted text
On 06 July 2018 at 12:51 "Ken Stone otherunicorn@... [cgs_synth]" <cgs_synth@yahoogroups.com> wrote:


You may have to try a lot of different transistors to get one that is noisy enough. I selected the 4250 because they were all noisy to the point of some of them being too noisy.
For transistors, the better the matching, the better they work - less bleed through. Try them and see if you are happy with the result.

Ken

On Fri, Jul 6, 2018 at 9:04 PM, Mike Burnham mikejburnham@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:


Hi there

For the Serge Noise circuit it specifies the noise transistor as 2n/PN4250 but these are stupidly expensive nowadays

Is it possible to use BC557? Are they noisy enough? BC557C probably best?


The VCA specifies a matched transistor package

Would matched 2N3904 be OK? Or do they have to be super matched?


Thanks






--
Ken Stone otherunicorn@...
Modular Synth PCBs for sale < http://www.cgs.synth.net/>




Re: A couple of parts questions

2018-07-06 by Mike Burnham

Can any small signal PNP be used?

Sent from my iPhone

On 6 Jul 2018, at 12:54, Ken Stone otherunicorn@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:

You may have to try a lot of different transistors to get one that is noisy enough. I selected the 4250 because they were all noisy to the point of some of them being too noisy.
For transistors, the better the matching, the better they work - less bleed through. Try them and see if you are happy with the result.

Ken

On Fri, Jul 6, 2018 at 9:04 PM, Mike Burnham mikejburnham@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:

Hi there

For the Serge Noise circuit it specifies the noise transistor as 2n/PN4250 but these are stupidly expensive nowadays

Is it possible to use BC557? Are they noisy enough? BC557C probably best?


The VCA specifies a matched transistor package

Would matched 2N3904 be OK? Or do they have to be super matched?


Thanks




--
Ken Stone otherunicorn@...
Modular Synth PCBs for sale <http://www.cgs.synth.net/>

Re: A couple of parts questions

2018-07-06 by Mike Burnham

Thanks for the heads up on AS394
Are these known to be good?

Sent from my iPhone

On 6 Jul 2018, at 13:32, JAMIE logan jammie.emma@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:

Show quoted textHide quoted text

using matched pair . 1/ there matched for you. 2/ because they are on same die the first thermal error is cancelled. which is perfect for vca.

3 / Erica synths sell the https://www.ericasynths.lv/shop/diy/diy-accessories/as394-matched-transistors/ for cheap.

time is money and matching transistors takes time.

the transistors ken chose for noise are good the other good one is a 2n5172 but pin out is different.


On 06 July 2018 at 12:51 "Ken Stone otherunicorn@... [cgs_synth]" <cgs_synth@yahoogroups.com> wrote:


You may have to try a lot of different transistors to get one that is noisy enough. I selected the 4250 because they were all noisy to the point of some of them being too noisy.
For transistors, the better the matching, the better they work - less bleed through. Try them and see if you are happy with the result.

Ken

On Fri, Jul 6, 2018 at 9:04 PM, Mike Burnham mikejburnham@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:


Hi there

For the Serge Noise circuit it specifies the noise transistor as 2n/PN4250 but these are stupidly expensive nowadays

Is it possible to use BC557? Are they noisy enough? BC557C probably best?


The VCA specifies a matched transistor package

Would matched 2N3904 be OK? Or do they have to be super matched?


Thanks






--
Ken Stone otherunicorn@...
Modular Synth PCBs for sale < http://www.cgs.synth.net/>




Re: A couple of parts questions

2018-07-08 by Andre Majorel

On 2018-07-06 11:04 +0000, mikejburnham@... wrote:
Show quoted textHide quoted text
> For the Serge Noise circuit it specifies the noise transistor
> as 2n/PN4250 but these are stupidly expensive nowadays
>
> Is it possible to use BC557? Are they noisy enough? BC557C
> probably best?

Not a month ago, Tony Allgood posted this to Analogue heaven :

Noise in reverse biased transistor junctions can be
surprisingly variable in quality. One of my designs, the 5U
Oakley Noise/Filter module, requires selection of the noise
transistor which is a BC547.

Originally I had thought that the process of selecting the
best noise transistor was to find the loudest one. But oddly,
the loudest ones actually didn't sound as good as the quieter
ones. The loud ones were often prone to random bursts of noise
while the quieter ones produced a more even and wide band
noise. The BC547 sounded better than the BC550 too.

Going by that, the BC557 would be worth trying. And possibly
better than the BC560, if whatever parameters distinguish them
have the same effect on the PNP as they do on the NPN.

--
André Majorel http://www.teaser.fr/~amajorel/

Re: A couple of parts questions

2018-07-08 by Ken Stone

Agreed. When I was shipping transistors with my PCBs, it was not the loudest that I chose, but rather the one that put the signal level where I wanted it.
Show quoted textHide quoted text
On Sun, Jul 8, 2018 at 3:01 PM, Andre Majorel aym-htnys@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:

On 2018-07-06 11:04 +0000, mikejburnham@... wrote:

> For the Serge Noise circuit it specifies the noise transistor
> as 2n/PN4250 but these are stupidly expensive nowadays
>
> Is it possible to use BC557? Are they noisy enough? BC557C
> probably best?

 Not a month ago, Tony Allgood posted this to Analogue heaven :

Noise in reverse biased transistor junctions can be
surprisingly variable in quality. One of my designs, the 5U
Oakley Noise/Filter module, requires selection of the noise
transistor which is a BC547.

Originally I had thought that the process of selecting the
best noise transistor was to find the loudest one. But oddly,
the loudest ones actually didn't sound as good as the quieter
ones. The loud ones were often prone to random bursts of noise
while the quieter ones produced a more even and wide band
noise. The BC547 sounded better than the BC550 too.

Going by that, the BC557 would be worth trying. And possibly
better than the BC560, if whatever parameters distinguish them
have the same effect on the PNP as they do on the NPN.

--
André Majorel http://www.teaser.fr/~ amajorel/




--
Ken Stone otherunicorn@...
Modular Synth PCBs for sale <http://www.cgs.synth.net/>

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