Dinky little 2-valve Chinese pre-amp

[Yorkie]

Surely the even-order harmonic distortion is the whole point of such a thing

Ah, the 'valve sound' Mark!

[Aerodyne]

So what sort of input is it designed for? I think line level, a CD or tape player, for example.
Tony

[Yorkie]

So what sort of input is it designed for? I think line level, a CD or tape player, for example.
Tony

Quoting from the spec Tony:

Item specifics:

Type: TUBE AMPLIFIER

Number of L/R RCA Inputs: 1
Number of Channels: 2
Number of Digital RCA Inputs: 1
Brand: LITTLE-BEAR
Audio Outputs: Stereo L/R RCA
Model: P-4
RMS Power: 20mw/per channel
Audio Inputs: Stereo L/R RCA

Hence, it doesn't mention the level of input called for - only the output. As you'll see, all it says about the input is that it has two RCA (phono sockets) for L & R channels! I'd imagine that even with the low HT, with a pair of twin-triode valves it will have reasonable gain.

My son intends to use it for a vintage 1970 Sony 'Optonica' RP114 record deck which he recently bought on e-bay as NOS - still boxed, and discovered along with lots of other stuff in a locked up neglected workshop. Not sure what the cartridges on it are. (There are two - they linear track each side of the record in sequence so there's no need to turn over the record).

I might add of course, that as Tony will know, there was a dinky solid state home-brew mono/stereo pre-amp design in Winter 2011 BVWS Bull, with full constructional details, PCB and so forth.

Oh well, my son's saved me etching a PCB for him!

[Refugee]

Is that not a voltage tripler behind the power in socket?
4 diodes plus 4 capacitors so 110V DC.

[Nick]

Is that not a voltage tripler behind the power in socket?
4 diodes plus 4 capacitors so 110V DC.

Don't think so - its a straight bridge with several small smoothing caps rather than one large one to:

a) lower the effective ESR
b) to keep the size down to fit the enclosure.

Thes valves will happily work at +12VDC so this one running at +28VDC (or whatever) B+ is well within bounds - its just the quality that suffers - their Ia/Va plot is not at all linear around that B+ and you need a Vg of close to 0V (which is an advantage here as you don't need a -ve C- supply.

[Aerodyne]

Yorkie wrote:

as Tony will know, there was a dinky solid state home-brew mono/stereo pre-amp design in Winter 2011 BVWS Bull, with full constructional details, PCB and so forth.

Yes, I vaguely recall that, David!
Looks like your little amp is indeed at line level and would require pre-amping for, say, a magnetic pick-up cartridge.
This is clearly a fun object with no hi-fi pretensions. Lighting up the valves with blue LEDs would tend to give the game away, rather...
Perhaps I've missed something along the way (not unusual). Assuming it isn't just for headphone use, how are the loudspeakers coupled? Is it capacitive, or if not, where are the transformers? And how much power output could one reasonably expect from such valves, driven as they seem to be by very modest HT?
Tony

[Nick]

Yorkie wrote:

as Tony will know, there was a dinky solid state home-brew mono/stereo pre-amp design in Winter 2011 BVWS Bull, with full constructional details, PCB and so forth.

Yes, I vaguely recall that, David!
Looks like your little amp is indeed at line level and would require pre-amping for, say, a magnetic pick-up cartridge.
This is clearly a fun object with no hi-fi pretensions. Lighting up the valves with blue LEDs would tend to give the game away, rather...
Perhaps I've missed something along the way (not unusual). Assuming it isn't just for headphone use, how are the loudspeakers coupled? Is it capacitive, or if not, where are the transformers? And how much power output could one reasonably expect from such valves, driven as they seem to be by very modest HT?
Tony

Its definitely line level - the 6N11 in this configuration has decent current gain - input sensitivity is likely to be around 200mV and with only 20mW/chan RMS out, its really just a line headphone amp/buffer - the output is capacitively coupled as per the schematic I posted above.

The blue LEDs are just a bit of fun - they make the valves look cool in some people's eyes - there are plenty of "high end" valve amps that do the same - its no indication of quality/lack of quality. This is done with nixie clocks etc. all the time (excuse the pun!) - orange (neon or heater glow) + blue (LED) is a decent combination...

Fun? Yes. HiFi? No.

[Aerodyne]

Told you I'd miss something - in this case, the 20 mW!
Tony

[Yorkie]

Thanks for your interest guys.

My son is using the pre-amp between his turntable and amplifier. I'm not sure what amplifier he has right now - I think it's a solid state one. He had a large valved amp with exposed valves etc on show - large bottles (can't recall the type), and a large mains transformer, choke and OPTs. I can't recall the make - it was modern production, seemed very well made and finished, had favourable write-ups in Hi-Fi mags, and looked very swish, (presumably from the Orient?), but he then sold it as it was really too much of a good thing for his needs. (I didn't ask how much it cost!).

When he brought it here over the weekend, one valve appeared faulty as whichever of the two channels that valve was fitted into, it didn't amplify yet the good valve would amplify in either channel. Although they'd supplied a spare valve, that seems not to be up to scratch either, so he's sent off for a pair of NOS replacements. I'll have to toddle over to Matlock when they arrive, along with my scope and waveform generator to see what's what. I don't think the amp is duff because as I said, either channel works fine with one of the valves. Time will tell.

[Nick]

A couple of points....

1. I'm amazed that it's sensitive enough to work with a deck
2. It has no RIAA equalisation so will sound odd.

The latter point obviously assumes that the main amp doesn't have RIAA as if it did, why would you use the little valve thingy in the first place?

I'm confused (not that difficult with me !)