Randall RG80/RG100 Preamp DIY

[nickbob00]

Hi guys

I am looking at the feasibility of cloneing the gain channel of the Randall RG 80 Or RG 100 series amps (to plug into a simple chipamp, <20W, maybe a TDA2003 for a practaise amp), however i find the way the schematic i found was drawn very confusing. I was hoping someone could help me interpret the circuit diagram. I am not a 'noob' and have built several electronic circuits before, so i dont need electronics 101. In case attatchments don't work, here is a link: http://www.schematicheaven.com/newamps/randall_rg100es-rg80es.pdf


I have a few questions:

1)Am I correct in thinking that the 'RED CHANNEL' is the distortion channel.

2)the circuit diagram says as a note 'All capacitors are rated in MFD at 100 volts'. I assume that means all values are in millifarads (as in 1000 uF?), but it seems strange to have a 10 mF (10,000uf) capacitor on the cathode of Q6?

3)D3 and D4 are captioned '4VZ'. Does this mean they are actually 4 volt zener diodes, even though they have the symbol of a normal Diode? A zener in the PSU is marked '12VZ'.

Thanks in advance!

[mensur]

I made that amp a few years ago and it's awesome.
Here are my samples:
http://www.esnips.com/doc/d6a08723-1fef-4ab9-afd4-f8520f273388/Randall-RG100ES-Boost-1
http://www.esnips.com/doc/489be47f-e102-49d0-aac5-b09ee8a4870b/Randall-RG100ES-Boost-2---Scooped-Mids
http://www.esnips.com/doc/619f7c75-6446-4b4c-85d6-6bd8258b2137/Randall-RG100ES-Boost-1(Solo)

1.Yes the red channel is the distortion channel
2.Caps are in micro farads (10uF on the source of Q6)
3.Yes they are 4V Zener diodes,(1N914 are clipping diodes,or boost in that amp, but they attenuate the signal a lot).

[nickbob00]

Thanks a lot for your help, Mensur.

May I ask how you constructed the amp - It would be great if you could post your layout for PCB/veroboard ?

Also, do you have a 'better' schematic (less confusing)?

Im starting a stripboard layout, but I will have to do a farnell order for an enclosure, the speaker and maybe parts for the power amp, if i dont use one of the TDA2003s i sampled a while back. This gives me an oppertunity to get a few PCB materials

[J M Fahey]

Hi Nickbob.
Yes, the schematic *is* complex, but you should split it into its "building blocks"
Re-draw only the Red channel , the tone control and the preamp output transistor.
No Reverb, Green channel , switching or power amplifier (loop is optional and can be added later) and work from that.
You'll have your layout work reduced to what's around Q5/6/7/4 .
That's what Mensur (and anybody) usually does.
You can provide your own chipamp and +24V.
Good luck.
Juan Manuel Fahey

[Zappacat]

I made that amp a few years ago and it's awesome.
Here are my samples:
http://www.esnips.com/doc/d6a08723-1fef-4ab9-afd4-f8520f273388/Randall-RG100ES-Boost-1
http://www.esnips.com/doc/489be47f-e102-49d0-aac5-b09ee8a4870b/Randall-RG100ES-Boost-2---Scooped-Mids
http://www.esnips.com/doc/619f7c75-6446-4b4c-85d6-6bd8258b2137/Randall-RG100ES-Boost-1(Solo)

1.Yes the red channel is the distortion channel
2.Caps are in micro farads (10uF on the source of Q6)
3.Yes they are 4V Zener diodes,(1N914 are clipping diodes,or boost in that amp, but they attenuate the signal a lot).

The first and last clips sound amazing.  What were you using for power in that amp?  Any schematics or layout?  Very nice work.  Thanks for sharing it with us.

[mensur]

Thanks,
The poweramp is my design with TDA7294(minor difference with datasheet's schem).Again the head has "LARGE" volume drop with diode clipping engaged(limit's the signal 0.7VAC p-p).

[mensur]

Here's new version of the preamp, with some new mods , MC34063 DC converter, to get 24V from 9V battery, plus it has voicing control, exact the same as Century C170II/2000,plus it has added gain stage to compensate volume drop caused by diode clippers.

[tra]

Here's new version of the preamp, with some new mods , MC34063 DC converter, to get 24V from 9V battery, plus it has voicing control, exact the same as Century C170II/2000,plus it has added gain stage to compensate volume drop caused by diode clippers.

Do you have a PCB layout or Eagle files for this?

[Gabriel]

HEy Mensur, nice job,
but I have 2 questions:
1 the original have 2 zeners where they go? Why u choose take off those diodes?
2 Can i put the tonestack of the Randall X2 in it?

[mensur]

Thanks,
The clipping diodes are in the circuit, before master contol.
It has +1 gain stage to acomodate signal loss after clipping diodes.
All Randall's have the same tonestack, Warhead has parametric mid control, which uses OpAmps, and I dont like opamps, so I did it like this.

[Gabriel]

Mensur, i would like to ask you again about the capacitors  c5, c8 & c14. Are they normal (film, poly etc ) or eletrolitical (polarized) ? Cuz their values are 1uF...

tankx for the previous answers.

[mensur]

Mensur, i would like to ask you again about the capacitors  c5, c8 & c14. Are they normal (film, poly etc ) or eletrolitical (polarized) ? Cuz their values are 1uF...

tankx for the previous answers.

I personaly like film caps(Wima MKS4) overthere, but you can try with elcos, and see what best suites you.

[kin0]

mensur can you tell how to use your schematic with the regular 24V? (What to erase and what to add?)

[mensur]

Here's what's new:
http://img826.imageshack.us/img826/4341/randallultimate.png

We have standard Randall RG100ES with contour control(which is from Century 200), which can be bypassed, plus it has additional gain stage for accommodating volume drop due to diode clipping.

[J M Fahey]

Congratulations, Mensur.
Excellent design.