Update :
I measured the maximum output level before clipping in Phono mode, @1kHz sinus of my MICRO-PREAMP : 9VRMS for 106mV input...
Compared to my Audio-Research SP3A1 offering 25VRMs, and my SRPP-ANZAÏ offering 50VRMS - both tube preamps - I was thinking about a rather low and cheap performance.
And then I read the specs of my McIntosh C712 preamp : 8VRMS. My NAD 114 preamp : 12VRMS. So at 9VRMS, the MICRO-PREAMP is not so bad, so is the 42-2109, its original circuit !
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I measured the maximum output level before clipping in Phono mode, @1kHz sinus of my MICRO-PREAMP : 9VRMS for 106mV input...
Compared to my Audio-Research SP3A1 offering 25VRMs, and my SRPP-ANZAÏ offering 50VRMS - both tube preamps - I was thinking about a rather low and cheap performance.
And then I read the specs of my McIntosh C712 preamp : 8VRMS. My NAD 114 preamp : 12VRMS. So at 9VRMS, the MICRO-PREAMP is not so bad, so is the 42-2109, its original circuit !
T
Just a question for the occasion :
What you you recommend for replacing the 2SC1740 transistors in this little circuit (Realistic 22-2109 phono preamp) ?
Not faulty indeed, but better ones, I mean.
BC237, BC547 and 2N3904 come to mind, but I'm far from a Solid-State ace...
Any suggestions ?
What you you recommend for replacing the 2SC1740 transistors in this little circuit (Realistic 22-2109 phono preamp) ?
Not faulty indeed, but better ones, I mean.
BC237, BC547 and 2N3904 come to mind, but I'm far from a Solid-State ace...
Any suggestions ?
If I were building new, I’d probably use the 547 series. They used all those Jap types because they were cheap and easy to source (for OEMs) back in those days. There was also a hell of a lot more selection - compared to lines being consolidated and low running parts dropped entirely. Nowadays the Euro and JEDEC types for the most part come out of Korea (and so do the KSC copies), some from China. Everything is about the same price now.
Would a BC547 be quieter? That’s the question. My gut tells me no, because the resistances in an MM phono preamp are high enough for Johnson noise to dominate. You can only get it so quiet, anything else is a waste. For an MC preamp, I’d at least be looking into component selection a little deeper.
If your transistors haven’t gone bad they probably won’t. The types I mentioned have a known reliability issue. Anything on that PROCESS did. RS used to use them a lot, including one PNP variety that shows up in power amp input pairs. They like to send DC spikes to your speaker - or at least make it sound like a thunderstorm in your living room (it could get that loud). They start out as some of the lowest noise types that were available in the 70’s, but they only sound like that when they’re new.
I’ve never known a tube to have that particular problem. Another black eye for transistors in the 60’s and 70’s.
Would a BC547 be quieter? That’s the question. My gut tells me no, because the resistances in an MM phono preamp are high enough for Johnson noise to dominate. You can only get it so quiet, anything else is a waste. For an MC preamp, I’d at least be looking into component selection a little deeper.
If your transistors haven’t gone bad they probably won’t. The types I mentioned have a known reliability issue. Anything on that PROCESS did. RS used to use them a lot, including one PNP variety that shows up in power amp input pairs. They like to send DC spikes to your speaker - or at least make it sound like a thunderstorm in your living room (it could get that loud). They start out as some of the lowest noise types that were available in the 70’s, but they only sound like that when they’re new.
I’ve never known a tube to have that particular problem. Another black eye for transistors in the 60’s and 70’s.
Thanks @wg_ski !
You are in the "if it works, don't fix it" about the transistors here, and you are certainly right, in tne sense that, according to you, other similar type for replacement won't do better (i.e. : BC107, BC237, BC547).
Note : I narrowed the types because of the 2SC1740 voltages specs at 50V/50V/6V, thinking that a 2N5088 and its 30V/30V/5V would not be convenient... But in that little preamp (Realistic 42-2109 base), it may be not important ?
Sorry I'm nearly a noob about transistors selection... 😕
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You are in the "if it works, don't fix it" about the transistors here, and you are certainly right, in tne sense that, according to you, other similar type for replacement won't do better (i.e. : BC107, BC237, BC547).
Note : I narrowed the types because of the 2SC1740 voltages specs at 50V/50V/6V, thinking that a 2N5088 and its 30V/30V/5V would not be convenient... But in that little preamp (Realistic 42-2109 base), it may be not important ?
Sorry I'm nearly a noob about transistors selection... 😕
T
50 volt types were used because the raw supply is 29, which could go higher. And the humbucker sees full voltage as the bypass caps charge. The time constant on the base does keep this sort-of under control, but I wouldn’t want to push my luck when 45+ volt small signal types with plenty of gain are under a dime if you buy them 100 at a time. Doesn’t even make sense to buy less. And already paid for even beats that dime. It’s a dime you get to use somewhere else.
OK - if it works, don't fix it ! The 2SC1740 works fine, and as you wrote it : If your transistors haven’t gone bad they probably won’t.
The MICRO-PREAMP was a bit hummy - OK, less than the original Realistic 42-2109, but... - So I added shielding :
Verdict : all hum eradicated. 😎
A Little-Cool, Hum-Free, Good-Sounding preamplifier, now... 🙂
The perfect mate for the MICRO-AMP, even if it's not a tube device ! 😉
T
The MICRO-PREAMP was a bit hummy - OK, less than the original Realistic 42-2109, but... - So I added shielding :
- grounded copper flux band on the bobbin of the transformer.
- 2mm aluminium separation screen.
- shielded cables for the Phono input.
Verdict : all hum eradicated. 😎
A Little-Cool, Hum-Free, Good-Sounding preamplifier, now... 🙂
The perfect mate for the MICRO-AMP, even if it's not a tube device ! 😉
T
Unfotunately, the flux band had no major effect, since it works against the radiation of the windings. That's why I added the Aluminium screen for working against the radiation of the laminations. Of course, shielding the Phono input cables helped...
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I would have thought “cheap high leakage reactance transformer, spewing radiation all over the place”. Guess they used a better one than I expected. Those ones they used to sell in the store put out enough stray flux to pick up at the South Pole.
Any transformer radiates, of course - more or less - the worse being probably the toroïdal, against all odds and by my personal experience... I have a R-Core, but never tested it : its construction inspires me even less confidence.
- If the mains is not perfect, flawless (it is likeky never, right ? spikes, DC offset, RF disturbances), chances are that the toroids spreads hum induction at 360°.
- If there's a slight overload : same thing.
- Plus they tend to buzzzzz...
A quiet, hassle-free toroidal transformer is a shielded / encased / dipped one, like the ones made by Toroidy, in Poland, for example, on their high-end range : https://sklep.toroidy.pl/
Regarding all this, the classic EI laminations transformer is much more tolerant... If it is correctly built, indeed. Moreover, the magnetic radiation is concentrated in the main field directions (core axis, plane of the laminations), unlike the Toroid which proves to be nearly a spheric radiator...
An example, on one of my DIY guitar amps :
First, I wanted to use a toroïdal transformer - I had two good one : a brand new, a spare for a 2x40WRMS COPLAND Hi-Fi tube amplifier, and another one, a bit larger. the COPLAND one even had a surround ribbon reputedly acting as a shield.
Toroïds are everywhere announced as low losses, lightweight, low radiations... Fine for a high-gain guitar amp, right ? Reality proved much different...
I installed the bigger one first :
A disaster, catastrophic : A General Hum Factory ! I though that I had made mistakes in the wiring arrangement ; GND loops, isufficient shielding... No way. How could this be possible ? Unacceptable.
Then I changed for the second Toroïd, thinking the first one, even brand new could be defective. The COPLAND is a Audio/Hi-Fi quality grade :
A bit better yes, but still unacceptable. De guerre lasse, I decided to remove that toroidal from the chassis, and draw the immediate conclusion that he was the culprit. Just surelevating from the (aluminium) chassis was already an improvement.
OK, let's surelevate and shield the unit, then :
Well... I understood that I wasted my time : another slight improvement, but nothing else. Hum was still unacceptable to me. Even the loudspeaker (EVM12L) still picked audibly the hum !
Then I called a friend who works in a winding and transformers company, makind EI, R-Core and Toroïds : he was not surprised at all, and went :
"Toroids are mainly interesting because they are smaller and lighter than EI, cost also less now, but they most of the time need the to be encased, to have a soft start and a DC blocker to perform as they should - at least over a certain power. For shielding, use thick aluminium screens, not steel. In your case, it will be as good as mu-metal, considerably cheaper and easier to use"
OK. Let's revert to a well-built classic EI transformer then, and add flux band plus 3mm aluminium screens or "wings" :
Tested stock, that EI transformer was already much more quiet, nearly acceptable, should I say... With the added flux band plus the aluminium screens or wings, it became completely quiet and hum-radiation free.
Voilà. Merci, Au revoir...
Then you understand the reason of the added 2mm aluminium screen in my MICRO-AMP aluminium box... I kept the small original transformer because it was absolutely silent, not heat at all, just as if it was powered off : this is a clue that it at least has been well-built, with low losses...
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