Tube sound from a transistor amp, it's possible

[sreten]

Originally Posted by Wavebourn

Nice amp. Class A 2-stage amp with emitter follower loaded on gyrator.

Hi,

I've never seen a gyrator load like this before, seen a choke obviously.
One does wonder about the size of the capacitor for the gyrator too.
(Unless its actually a variation on the White cathode follower.)

rgds, sreten.

I said about 3W based on the 15V supply shown. 23V allows more.

[DF96]

It looks like a simple bootstrapped load to me. The drawing puts the components in unfamilar places, so the circuit operation is somewhat obscured.

[JMFahey]

Agree.
I see no gyrator there, but a simple single supply single transistor front-end amplifier.
I remember when they were "state of the art", go figure

[wahab]

Typical of some Grundig portable radio schematics drawings of the amp part , it looks
like they are doing their best to make you follow legnthily every single path
to check if it goes effectively where it is supposed to , indeed this is the same
schematics , wich apart from this originality is a typical CFB amp with
a simple complementary output pair , with of course a bootstrap that is more
than necessary given the low current gain of the final stage as well as the limited
VAS current , on the order of 10mA in this case , wich will likely make the output
stage run out of gain at high level , surely an important parameter in the final sound.

[EDDELARUE]

VAS current ~ 1mA

[wahab]

The VAS current is defined by its loading resistors , 390 + 220 = 610R
with about 7V to shift , the current is 11mA.

[EDDELARUE]

Originally Posted by wahab

The VAS current is defined by its loading resistors , 390 + 220 = 610R
with about 7V to shift , the current is 11mA.

Ops! You're right !

1mA is on the first stage

Sorry.

[Loudthud]

Originally Posted by repair guy

My mistake, the diode is supposed to be two diodes in series. The 10 ohm resistor in the feedback circuit is right. The 47K on the capacitor is 47,000pf. The amp has alot of gain, has some ringing on square wave test, four bumps on the leading edge. This leads me to believe that the neg feedback can't be increased much. It does oddly manage to have almost no crossover distortion (can't even see it on the scope). 3db points are 80hz flat to 50khz, with usable bass down to 55hz (possible predistortion in preamp). Power supply is around 23v. There's no way it's only 3 watts, has to be at least 7 per side (it can shake the floor). It's not a serious amp power wise, it's just the only transistor amp I've ever heard that sounds like a single ended tube amp. When under 2 watts it just sounds so clear, it kind of baffles me how such cheap components, weird "or poor" design choices can come together and sound really good. I just posted it to see of the topology of the amp could be built into a more powerful amp. I just figure that if such poor components can sound good, then better parts and fine tuning of values might lead to really great sounding amp. There's something to be said for a design that has a gradual increase in distortion with power. But I do consider this amp to be hi-fi, it's just not going to rock the house. Most of the time I listen to music under 5 watts anyway, if I want loud I use a more typical transistor amp. It can go loud, then it's bass gets really sucky.


Keith

Take a look at the "Amp Camp Amp" in the Pass Labs forum. It's a simple MOSFET class A power amp that runs off of a 19V laptop power supply. Disconnect the 68.1K feedback resistor and it sounds more like a tube amp than some tube amps. The only things missing are the saggy power supply and the noise.

[Wavebourn]

Originally Posted by DF96

It looks like a simple bootstrapped load to me. The drawing puts the components in unfamilar places, so the circuit operation is somewhat obscured.

You are right, I missed the diode.
No gyrator, just bootstrapped complementary emitter follower. My imagination improved it a little bit.

[destroyer X]

Depending second and third harmonics results i will assemble to listen.

It is interesting....and if musicians said it sounds good..then i want to check that.

These guys that works with music usually have excellent ears...they are highly skilled related sonic evaluation...well...this is what they do all time long.... i do respect them and i do not discuss with them because for sure they gonna win the discussion in the end of the tests.

regards,

Carlos