Bipolars for low noise input - Alternatives to 2SC2240 - Page 6 - diyAudio
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Old 28th February 2011, 07:43 AM   #51
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Quote:
Originally Posted by masag1 View Post
Hallo Hartmut,

If you need 2SA1316 and 2SC3329 for testing, please send me a PM.

With kind regards.

Sam
Hi Sam,

Thanks, email sent.

This is the current list of bipolars tried these days in above configuration, in order of personal preference, best liked on top:

2SC2546E
2SD786R (cpl. to 2SB737R)
LM394CH
MAT02FH
2SC3381

I just changed back from 2SD786R to 2SC2546E, to experience the difference, they are quite close. The 2SC2546E is a touch more transparent, while the 2SD786 has a bit more bass grip, but sounds a bit grey in midrange compared to the other.

After trying some two other transistors, I will start to develop the circuit:

- symmetrical loading of input transistors for lower offset (Early voltage!), there is 0.6V difference in Vce between the input transistors currently
- either folded cascode for simple moving coil stage
- or differential with some gain, also some degeneration, for first stage of a complete phono stage
- second stage (be it cascoded or differential) loaded by current mirror, with output load resistor, eliminating the output cap, with DC servo.

BTW, EMT uses the LM394CH in symmetrical common base configuration in the moving coil stage for their EMT 938 turntable. EMT fans prefer step-up transformers in this place, though.

best regards,
Hartmut

Last edited by hifidaddy; 28th February 2011 at 07:48 AM. Reason: corrected error
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Old 1st March 2011, 08:37 AM   #52
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by hifidaddy View Post
.............I started with 4 Volt rails, instead 12V, and gradully increased the voltage. At all voltages, I measured the voltage offset at the emitters of the two input trannies. Unfortunately, at rails > 8V, there was an increasing offset > 50mV, so did not dare to use this with my delicate Ortofon MC7500 cartridge.
.....
This is a shocking high value. Even 5mV would be extremely high. Given the circuit, it seems strange that such a high voltage can develop between the two emitters.
Looking at the picture, i can see that you did not thermally couple the input transistors. Also, you should match them for beta and Vbe at the current you are using them in the circuit. Use low tempco emitter resistors.

Check this thread, what John Curl and others are saying about a similar approach: MC pickup under current...how much is OK?
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Old 1st March 2011, 11:58 AM   #53
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yeah, the offset voltage is something to consider. At 6V rails, and with 0.6Volts offset at the collectors, there is about 5mV Vee at open inputs. When a cartridge has been connected, this voltage divides between 2 times Re and Rcart. In my example 2*13 Ohms (at 3mA Ie) and 6 Ohms of the Ortofon MC7500 catridges. So only 1 mV of the initial offset is present at the cartridge leads, leading to 0.16mA bias current through a 5000 DEM (3000 USD) top of the line MC cartridge. This should not fry the coils. So far the results are very good, subjectively. If the subjective sonic results will improve, once this offset voltage and current have dissappeared, will be be interesting aspect of further experiments.

Hartmut
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Old 2nd March 2011, 01:33 AM   #54
Salas is offline Salas  Greece
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Originally Posted by hifidaddy View Post
Hello Ruediger,

I have many years ago tried SK170/240/146 and the like in common source configuration, with a resistor as load, no cascode at all ... among these usual suspects, I liked the 2SK97 best (see the 1995 Kaneda preamp), but what I missed with all these JFETs was naim style tightness, grip and PRAT. Since I had some lucky years with naim and naim copies in 80ies, I got used to what naim does right, and I am reluctant to pass on this (no pun intended!).

Though, as you mention bass tightness with your proposed configuration, I think I will give it a try this week. I still have a JFET moving coil stage here, have to add a cascode only.

Thanks a lot.

Hartmut
Since you liked Naim, I have done an overhaul of NA323 on simulation only, with easy to get, cheap transistors, but the parallel quintet input stage (biased at 1mA each in this version) can host Hitachi, Toshiba, and ROHM also. R1 is the cart's load, can change value of course. With this is I wanted to modernize the classic towards a much smoother HF response, more bass extension, more bandwidth, more gain, and far less input noise. Also I realigned it so it can use practically small coupling values and be able to work with film capacitors and not those trademark bottleneck tantalums. Maybe its interesting to you, I don't know. I have in mind to try it at a point too. It looks solid on the simulator but nobody knows if it will need some debugging in practice. Best of luck with all your experiments in any case!
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Old 2nd March 2011, 07:32 AM   #55
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Salas,

this was exactly, how I started DIY in mid 80ies. My first "project" was a DIY variant of LM317 equipped Naim power supply (Snaps, Hicap), with two regulators in series, like Naim realized many years later in the Supercap. My changes to the naim circuits itself have been less successful, maybe naim had found the single sweet spot for this vintage circuit.

I focussed on differential inputs in preamps later, and my power amps are not that far away from the AKSA or destroyer circuits known in this forum.
Well I want it all, naim grip, but also low distortion, max transparency, fine detail at the same time.

There are some DIYer who make alternate boards for Naim, either commercial or group-buy folks, you can meet them on the pinkfishmedia forum. IIRC, the "Starfish" is a state-of-the-art version of the naim line stage. Today I don't care about bodging with naim any more, just have some of the cute half width vintage naim components in the living room for family.

Hartmut
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Old 4th March 2011, 07:52 AM   #56
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Finally, I had to install my long-time favourite 2SC2240, and also the 2SC3329 that Sam had sent to me (thanks a lot!).

So in the end, I found an alternative to the 2SC2240. Ironically, this is not a special low-noise (low en) transistor.

Upated current top list, best liked on top

2SC3329
2SC2240
2SC2546E
2SD786R (cpl. to 2SB737R)
LM394CH
MAT02FH
THAT300
2SC3381

Hartmut
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Old 4th March 2011, 08:06 AM   #57
Salas is offline Salas  Greece
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2SC3329 is very low noise. 0.6nVrtHz 2 Ohm rbb'.
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Old 4th March 2011, 09:02 AM   #58
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Originally Posted by Salas View Post
2SC3329 is very low noise. 0.6nVrtHz 2 Ohm rbb'.
oops, you are right, I have mixed up some datasheets.
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Old 29th March 2011, 08:48 AM   #59
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I got too many things to try in my setup (different horns, woofers, tonearms), so I kept this moving coil stage constant for several weeks.

Though when I got a currently hyped moving coil cartridge for a week of home testing, and suspected a frequency anomaly, I took measurements of this moving coil stage with cartridge, using the Hifi News + Records test LP pink noise track, and had a look at the ARTA spectrum analyzer at line level. For the pink noise slope I made a compensation file.

I found an enormous deviation in the FR compared to high-Z loading, transformer loading, or 47k loading (Moving Magnet input).
With an operating point of 3.5mAmps each transistor there was a recessed midrange of about several dB compared to bass, and then a rising output above. The total deviation was 40dB. At 1mAmps each transistor the deviation went down to 30dB.

I also checked another cartridge, and coincidently, it showed exactly the same characteristic.

So I have to conclude, that zero-impedance or even low-impedance loading is not applicable to most MC cartridges.
While the sonic results are very attractive, and even visitors had positive reactions during the time I had this MC stage running - the deviation came not as extra nasty treble, but as extra detail and transparency - I make a stop here, and I have to change my moving coil stage in order to make it measure with MC catridges and sound good at the same time.

I will add some FR graphs later.

Hartmut
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Old 29th March 2011, 07:31 PM   #60
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Hi Hartmut,
are your really saying that the treble FR is 40dB above the bass region? That would be a riaa-pre with, well, without RIAA-correction...

Rüdiger
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