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Old 1st January 2013, 06:20 AM   #1
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Default Tube sound from a transistor amp, it's possible

I would like someone's input on what kind of amp this is. This amp I've had for a long time. Looks like a piece of junk, tiny power transformer, only 10 watts per channel, made by mayfair (the tiny tape recorder company). I work on amps for a living and have heard hundreds, and my shop is in a music store, so get to hear LIVE instruments on a daily basis and I'd like to think I have a good idea of what sound should sound like. But this amp has something that Is so interesting and cool, it sounds like a tube amp. I've demonstrated this to several people "mostly musicians" and they think the same, I've even tricked people by hiding this amp, a couple cables, a decoy tube amp head turned on "standby". They reach for the controls and "whoops" they don't do anything, because the little mayfair is wired up instead. They are surprised, or confused, but they all say "that's a darn good sounding amp", or they think the mayfair has tubes in it.

I use it for music, not guitar, and it just sounds so good, very nice in the midrange, sounds like you can pick out each instrument in a song on it. The tradeoff is it's not that good in the bass. I'm more of a repair guy, not a designer, I know there's people on here that could give me some insight on the circuit, it's very simple. Only four transistors per side, outputs look like big TO92's, and are pretty fast in the Mhz and are complementary. I think it's a type of complimentary - symmetry circuit. It's 41 years old and all original and it can crank, giving that's a small room and it's hooked up to a couple of efficient speakers. I've driven it hard, but it doesn't clip, it starts to compress the sound if pushed too hard, exactly like a tube amp would. What in the circuit makes it do that? What is the 200uf and 10ohm on the 2sa628 for? Or any other details would be appreciated.

Thanks,
Keith

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Old 1st January 2013, 06:46 AM   #2
Mooly is offline Mooly  United Kingdom
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The 220uf, the 10 ohm and the 3.3k are the feedback components. Is it 10 ohm or 100 ?

The amp is a simple single ended input design (which is a major part of its success sonically) together with a simple class b fixed bias output stage. Increasing the value of the electrolytics in the signal path would improve the low frequency end dramatically. 470 uf for a speaker coupling cap nowadays (modern speakers and sources) is to low by a factor of at least 5. Also 40 year old caps can't be up to snuff today.
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Old 1st January 2013, 05:31 PM   #3
DF96 is offline DF96  England
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Looks like a 1960s circuit, even if it was actually made in the 1970s. The relatively low amount of feedback will mean it clips less aggressively than modern designs, but below clipping it will have much higher distortion. Crossover distortion may be a problem with such a crude bias mechanism. It certainly won't sound like a decent tube amp (whch won't sound of anything).

I would regard it as a historical curiosity, as it appears to be neither a guitar amp or serious hi-fi.
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Old 1st January 2013, 05:59 PM   #4
cotdt is offline cotdt  United States
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Looks promising. It would be interesting to build one with modern parts.
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Old 1st January 2013, 06:12 PM   #5
willamp is offline willamp  United States
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With a 15v supply how could it be any more than 3.5W into 8ohm or 7w into 4ohm?

7.5vpeak X .707 = 5.3v RMS

5.3^2 / 8ohm = 3.5w

Is my thinking flawed? Power rating is always based off of RMS volts, isn't it?
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Old 1st January 2013, 06:16 PM   #6
cotdt is offline cotdt  United States
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3.5W/7W can get pretty loud indoors. You can build a modern version and up it to 30V rails quite easily for more power.
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Old 1st January 2013, 06:39 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

Seems very typical for the low power amplifiers built into tape recorders,
and the restricted output cap value probably relates to the small tape
recorder drivers fitted, no point sending bass they can't handle.

Would have to model it in the free TinaTi as I don't
intuitively understand the circuit and whats going on.

Feedback gain seems far too high, unless I've missed something.

The JLH and DoZ are the accepted "valve-like" transistor designs.

This one has much less power and needs a thorough analysis.

10W per channel is impossible, more likely around 3W.

rgds, sreten.
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Old 1st January 2013, 07:25 PM   #8
DF96 is offline DF96  England
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If it was a modern amp then the quoted 10W would be Chinese watts. Back in the early 1970s they may have been Hong Kong watts or even Japanese watts.

The main LF limit is not the 470uF output cap, but the 220uF feedback cap - about 70Hz rolloff? One wonders about the hearing ability of the 'musicians' who like this amp.
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Old 1st January 2013, 09:09 PM   #9
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The choice of transistors in it are what baffles me. Either they used what they could get cheapest or they are leftovers, but there would have easily been better choices available.
2SA628 25V(vce) 100mA(ic) 100Mhz(ft) hfe 100-800 depends on suffix
2SC711 25V 50mA 75Mhz hfe 300 max
2SC1209 20V 500mA 75Mhz hfe 35 min
2SA695 20V 700mA 75Mhz hfe 50 min

DF96 these are surely Japanese watts since the data for these transistors are only available in Japanese as well.

Last edited by jerluwoo; 1st January 2013 at 09:12 PM.
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Old 1st January 2013, 09:42 PM   #10
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by DF96 View Post
The main LF limit is not the 470uF output cap, but the 220uF
feedback cap - about 70Hz rolloff? One wonders about the
hearing ability of the 'musicians' who like this amp.
Hi,

10R can't be right for the feedback cap, unless I've missed something.
I'm assuming its a 100R and then the 220uF doesn't set the LF limit.

rgds, sreten.
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