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Author Topic: Vintage preamp and hiss  (Read 7001 times)

Julien Asencio

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Vintage preamp and hiss
« on: October 12, 2011, 12:33:04 PM »

Hi, I have some hiss in my recapped preamps based around bc109
They are old (late 60) and I wonder if the hiss I have (at high gain)
would come in particular from the resistors?
Change them to new ones would be a improvement? What do ya think? Thank you ;)
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Jim Williams

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Re: Vintage preamp and hiss
« Reply #1 on: October 15, 2011, 04:38:25 PM »

The resistors are carbon film so they don't add that much hiss. Only the front end resistors are likely to make any noise differences. Use Dale CMF60's or similar. It also looks like there are some box resistors like used in old U-87's, change those to metal film.

BC109 metal cans are worth $$$ on the Ebay market. Vintage fuzz box makers are looking for them. New versions are plastic TO-92's.

The BC109's are not considered a low noise bipolar transistor today. They are general purpose. To lower semiconductor noise use lower noise speced parts. For NPN's the Renesas (formerly Hitachi) 2SC2545 are quietest along with currently made Toshiba 2SC3329BL.
Pinouts are ECB on the jap parts.
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John Roberts {JR}

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Re: Vintage preamp and hiss
« Reply #2 on: October 16, 2011, 12:24:44 PM »

+1 (mostly) to JWs comments.

While the amount of potential benefit is dominated by the design, the most likely area for improvement is substituting quieter modern transistors, for the very input pair. Transistors later in the circuit will have a much smaller impact on overall noise.

All resistors make the same basic noise for the same resistance value and temperature (Johnson noise), so the only improvements available other than changing the design (value), is reducing secondary noise effects from current flowing and granularity of the actual resistance (excess noise). This is not generally very audible but if you have money to burn, it won't hurt anything to improve that.

JR
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Jim Williams

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Re: Vintage preamp and hiss
« Reply #3 on: October 18, 2011, 11:48:17 AM »

Since there are not that many parts, you can experiment with replacing all the resistors with something like the Dale CMF60's.

Resistors unfortunatly don't follow the ideal on a nomograph noise chart. Materials do affect performance and results. The 1/2 watt MF resistors do spec a bit less noise than their 1/4 watt counterparts, even if the same brand.

I also find the tops to sound a bit more open and clear with those quality resistors over carbon films.

Improvements in capacitor construction should also be considered over replacing older axial caps with newer axial caps. The new Panasonic FR series has the lowest impedance out and 10,000 hour life at rated temco's. Their case size is also reduced over the FM series. 330 uf/25v is the same size as the 220 uf/25v FM series. A small film cap bypass can reduce the filtration of the small transient waveforms that are absorbed as heat via dialectric absorbtion in electrolytic caps.
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John Roberts {JR}

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Re: Vintage preamp and hiss
« Reply #4 on: October 18, 2011, 06:37:14 PM »


 A small film cap bypass can reduce the filtration of the small transient waveforms that are absorbed as heat via dialectric absorbtion in electrolytic caps.

That isn't quite my understanding of DA, and in my bench tests (a few decades ago) to make big caps act like small caps it took more than just a small cap in parallel. I had to use 10% of the big cap's value to effectively manage what was probably ESL (I was measuring HF phase shift in a LF RC circuit). I've never found DA very much of an issue in high impedance DC blocking circuits, with poles tuned down in the dirt.

JR
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Jim Williams

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Re: Vintage preamp and hiss
« Reply #5 on: October 19, 2011, 12:09:37 PM »

DA and ESL are interactive. The ESL resistance of those high frequency details are converted to heat, then becoming DA.
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John Roberts {JR}

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Re: Vintage preamp and hiss
« Reply #6 on: October 19, 2011, 05:54:39 PM »

DA and ESL are interactive. The ESL resistance of those high frequency details are converted to heat, then becoming DA.

Wouldn't resistive elements in DA be more a ESR than ESL component?

However the distributed Rs in DA are shunted with distributed C, which is why DA is a delayed output phenomenon since the distributed Rs and Cs prevent the cap from fully charging or fully discharging instantly when driven from a low impedance. This is a real problem for sample and hold circuits driven from a low Z, but for DC blocking caps where the termination is relatively high impedance and terminal voltage is relatively unchanging, the cap is probably fairly characterized by ESR and ESL for transient events.

It has been many years since I invested bench time into this pursuit (caps), it seems this should be easy enough to see and measure with null testing. I guess you could null test between good and bad caps with music as a source to see where the errors you hear are occurring.

A null test between caps, with small helper caps in parallel, compared to a straight piece of wire (or big film cap if that's too hard), would answer my question about how big the little helper guys need to get to be effective (to act like a big film cap).

When you get your caps to null out against wire, they should be good enough to even make golden ears happy.

JR
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Jim Williams

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Re: Vintage preamp and hiss
« Reply #7 on: October 19, 2011, 08:04:23 PM »

Sorry, meant ES "R", not the L component.

Kidbill asked if the transistor swap will change the tone of the design.
Possibly. If the GBW of the parts are similar, they should sound close, but individual parts can also vary and also the source of them.

As I mentioned, those older metal can BC109's are collected by fuzz tone makers for a particular characteristic. They are very common in old German audio gear, Studer 069 consoles and other stuff. If you want to experiment, install machine transistor sockets in one module. Then you can play with different parts and discover any differences yourself without committment first. Once you make your selection you can solder in the rest on the other modules.
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Jim Williams

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Re: Vintage preamp and hiss
« Reply #8 on: October 26, 2011, 11:21:26 AM »

On a similar front I just rebuilt a Echoplate preamp. It used a 2N4222 jfet into a 2N5087 bipolar transistor, then into a 351 opamp and current booster.

I replaced the resistors with the Dales and installed LSK170 jfets and 2SA1084 bipolar transistors. The opamps are all LME49710NA's.

Noise is WAY down now, with the high output pickups it should do about a -95~100 db s/n, pretty good for a plate reverb.
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Julien Asencio

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Re: Vintage preamp and hiss
« Reply #9 on: October 26, 2011, 02:00:21 PM »

Thanks!
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