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First shot at a battery DHT preamp stage
First shot at a battery DHT preamp stage
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Old 6th November 2019, 07:57 PM   #51
piano3 is offline piano3  United Kingdom
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These days, I do all my small signal stuff with transistors but I still use decent voltages, up to about 150V (although I do realise this is overkill). It really gives you options to keep the distortion down.

I saw a circuit somewhere for a Theremin
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Old 6th November 2019, 08:34 PM   #52
mondogenerator is online now mondogenerator  England
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I havent done a huge amount of audio things with discrete semiconductors - though more than with valves.

I made Rod Elliot's P3 amplifier, using MJE3055/2955, with MJE340/350 as drivers, rather than BD139/140. Not small signal though

Most of anything using discrete components has been simple, and generally instrumentation, front ends for H bridges.

But the datasheet are easier to come by, and read.
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Old 6th November 2019, 09:01 PM   #53
mondogenerator is online now mondogenerator  England
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I think theres a theremin, and loads of other interesting musical circuits, on the experimentalists anonymous site.
Theres so many I havent begun to scratch the surface
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Old 7th November 2019, 01:16 PM   #54
Lampie519 is offline Lampie519  Netherlands
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A Theremin?
Something like this...?
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Old 7th November 2019, 04:51 PM   #55
mondogenerator is online now mondogenerator  England
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Yes Lampie,
A Moog as well! I would bet that it's a little less like the more familiar Theremin tone.

I saw a demo of one of those, and it sounded decidedly "modern"

I'm sure they have more than one Voice however
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Old 7th November 2019, 06:00 PM   #56
Lampie519 is offline Lampie519  Netherlands
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Yes, you can create a lot of noise with these haha, Moog or not.
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Old 7th November 2019, 07:36 PM   #57
mondogenerator is online now mondogenerator  England
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Haha I joke Lampie (my jokes are poor, but the medium of humour is poorly conveyed here, in text)

I would be happy with almost any Moog
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Old 13th November 2019, 01:35 PM   #58
mondogenerator is online now mondogenerator  England
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Default Quick lunchtime test and update

I grabbed 10 minutes to set up and run a quick test of the simple DH Triode strapped 1J29B circuit.

HT 90V
Va 50V
Vg -3.3V (2 series LED string)
A ~5 V/V
Rload 100k

Signal input of up to 1 Vrms

A quick glance at the FFT and it seems the circuit is performing OK

Opinion welcome, vis a vis usual OL THD vs Gain)

I see H2 down at -40 to -50dB or more (signal level dependant), H3 down another 10 to 15dB.

Higher harmonics down in the noise floor at or below -60dB.
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Old 18th November 2019, 05:41 PM   #59
mondogenerator is online now mondogenerator  England
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I got some more play time today, so I switched out the LEDs and substituted a diode string using 1N4007.

Due to the nature if these valves, or my circuit, I have found that biasing them so the Anode is slightly above or below 1/2 B+ gives me the best gain, and also THD.

As Piano3 or Lampie mentioned, the valves seem to perform best with about 1.5mA of quiescent current.

Anode load of between 27k and 39k, using between 5 and 7 series connected 1N4007, for Vg -3V to -4.2V respectively; seems to work well.

Also, connecting G3 to K, or A when Triode strapped, makes little difference, certainly nothing I can measure.

Gain is between 5 and 6 V/V depending on Anode load etc, due to the slightly different operating points mentioned above.
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Old 18th November 2019, 05:49 PM   #60
mondogenerator is online now mondogenerator  England
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Quick test in pentode mode.

(Without changing anode load, which I forgot, and should've done)

B+ 90V
Va 50V
Vg3 = 0V
Vg2 68V tapped off B+ using 100k, 4uf bypassed to 0V
Vg1 5 to 7V using LEDs and diodes.

Gain (in the messed up example) about 23V/V
THD similar to triode strapped, at the same dBV output level (input signal to get there is a lot less, obviously)

Both measure about 1% (Open Loop)
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