Banggood "Tube Amp 2" Kit

[Mark Hennessy]

Hi Ken,

Yes, there is a 20 minute edit window. That's pretty standard across forums. When Golborne first started, we ran without this restriction, and found that it occasionally caused problems.

This is an example. If you amend the original post, some of the subsequent posts that question the valve type will make no sense!

Naturally, I could edit your review, and am very happy to do so. So once we're 100% sure what this valve is, I'll update your review, but will also need to edit or delete the subsequent posts that question the valve type.

Thanks for the review, BTW. It's an interesting looking bit of kit. Shame there's no schematic. I'd be interested to know why they're combining 5532s - one of the best, lowest distortion audio op-amps ever produced - with valves. Often the valves are just followers - sometimes they're not even in the signal path! We don't just listen with our ears

Mark

[pwdrive]

http://www.analogmetric.com/goods.php?id=387

Lawrence.

[ppppenguin]

That's great. 2 utterly different valves sharing the same type number.

[radioman]

Found similar looking kit on Aliexpress with a bit more info. including circuit diagram.
https://www.aliexpress.com/item/Class-A-...45073.html


Andy

[pwdrive]

Maybe electrically it's the same...who knows?

They appear to come from the Shuguang factory.

Out of interest here's a link to a copy of the Shuguang Tube Manual (1973):

https://frank.pocnet.net/other/Shuguang/index.html

Lawrence.

[Mark Hennessy]

Thanks for the link, Andy!

So the valve appears to be doing something. But the op-amps are doing the real work. The NE5532 is known for being good at driving low impedance loads while maintaining low distortion, so it's not surprising to see them there.

Any guesses as to what the valve is actually doing? Is it a follower, or is it providing gain? If the latter, then it should be the "nice" sort, with the 2nd-order distortion that we love so much

[ppppenguin]

Difficult to tell from that circuit but I reckon it's a follower. The output is from 2 electrodes that are connected to the -ve rail via a resistor. Hence likely cathode and suppressor grid.

With 100% -ve feedback the valve won't be doing anything much to give "valve sound".

[Mark Hennessy]

Quite.

I was leaning towards follower, given that the op-amp section has 10dB gain.

[ppppenguin]

The NE5532 is in an odd arrangement too. First stage is giving approx x3 or 10dB gain as Mark says. This could drive the output directly but there's another stage as a voltage follower. The strangeness is the pair of 47R resistors with themain output and feedback for the first stage taken from their junction. I just don't see what they are trying to do.

Other oddities include R7, 1M which does nothing.

[Mark Hennessy]

It's to double the current drive available. Some headphones are 32 ohms...

Taking the negative feedback from the junction of the two 47 ohm resistors ensures that the amplifier is a good voltage source. The two 47 ohm resistors ensure the two op-amps share the current nicely. Theoretically they provide current limiting, but the op-amps themselves are no-doubt doing that themselves.

It's not a terrible circuit, despite appearing to be unconventional. Well, the op-amp part, at least - I can't really comment on the valve bit.

If you have a couple of spare tubes of NE5532s, you can scale this up to drive loudspeakers. Douglas Self published a design that used 32 of them!