JCM800 Emulator to bipolar supply

[MrStab]

hi,

as no-one's asked me to make them anything this week and i'm a compulsive sort of person, i've decided to try making a simple solid-state amp. i have myself a transformer from an old practise amp (with an unloaded reading of +/-13.8V), and plan to use a TDA2030 recycled from the same amp for the power amp.
i'm being extra-cautious and paranoid as i know from experience that an AC handshake isn't too enjoyable a sensation.

anyway, i'm a Marshall kinda guy. i'm just cliché like that. so i thought i'd get started by making Electric's JCM800 emulator as the preamp. i've recently converted a bass preamp FROM bipolar TO single-rail (thanks to the guys on my Rusty Box thread), but now i want to convert a single-rail circuit TO bipolar.

the ground becomes the negative rail, i know that much (or does it..?!). what about biasing etc? in the aforementioned conversion, vbias became ground, but i'm unsure what to make ground in the JCM800 emu (schematic here: http://i1236.photobucket.com/albums/ff443/gregwbush/jcm800emulationSchematic-electrictabs-addedoutputcapbyFlo.png). any ideas?

surprisingly, the power supply & power amp sections i have in mind seem to be less-confusing than the preamp situation!

cheers loads for any help

[PRR]

It's a single 9V supply.

And a distinctly single-supply circuit. (Unlike your power amp which derives some benefit from supply symmetry.)

Derive +9V from your large positive rail and be done with it.

Because it uses the drain-trim instead of source trim, the power suckage could be 2mA to 25mA, and will vary in trimming. So you can't do the simple thing: a voltage-drop resistor. You could run a resistor to a 9V Zener to pass >30mA no-load, but that's a hot Zener, especially since your raw voltage will vary a lot between wall-wander and power amp surging. So I guess a 7809 or similar.

[MrStab]

Derive +9V from your large positive rail and be done with it.

that had not even occurred to me.

so from what i gather, ground is ground throughout all three sections (post-transformer). a 7809 would be my preferred route, i have a coupla 9.1V zeners on hand but i've seen them readily melt, particularly when used in series. i'll need to get a bunch of regulators for when i kill the first few with the lack of a heatsink... (will be sinking the TDA ofc)

thanks!

[duck_arse]

melting zeners, there's something I've never seen. sounds like they mave have been pointing the wrong way.

[Bill Mountain]

My only thought is...do you know the input impedance of the power amp or does your preamp have a buffered output?

[PRR]

> 9.1V zeners on hand but i've seen them readily melt

Do you know what you are doing? Zeners need resistors.

[J0K3RX]

um... pretty sure that JCM800 preamp will run on 18v if your caps are rated as such...

[MrStab]

Do you know what you are doing? Zeners need resistors.

this was back before i had actually read an article informing me of such, but for all intents & purposes, i probably still don't know what i'm doing lol. i've successfully used other values of zeners since, thankfully. just thought it was worth mentioning as i hadn't seen other diodes do this, but then i guess they have different applications. i thought it was a "known" thing, evidently it was just my stupidity.

after some frustration with the JCM emulator, on a whim i decided to go for an Xotic BB Preamp with a clean (ie. no diodes) option and a slightly-modded LPB1 gain stage afterwards (1uF caps & 2.2k instead of 390R to NPN Emitter). as a preamp, it sounds pretty damn badass. now i'm just having trouble with the power amp - both circuits work by themselves, but my transformer sags like crazy when both are hooked up - i think i need to score some 1/2W 100R resistors for the power supply filter (as opposed to the 330 i'm using now) and my TDA2030 might be dead. not sure if this is due a different thread or not. i'll do some reading, just not sure where to start looking. http://sound.westhost.com/project04.htm is a good link.