Combine external with internal power supply in “separate” parts ofcircuit combo? - Page 1
Forum section: electronics / projects

author: tedsorvino
author_rank: Regular Contributor
timestamp: July 12, 2025, 07:03:53 pm
content: Hi everyone.I’m in the middle of modifying an old solid state FBT preamp-tape echo unit. The original had a solid state power amp involved, since it was an old PA combo.I only use 2 of the input preamp circuits, the tape echo circuit (playback and recording amplifiers, bias oscillator etc.) and the mixer circuit as a mixer for the effect and the dry signal and output.The power supply provides 2 dc power rails (along with some ac rails relevant to the tape echo motors and voltage meters).1 x 70 volts dc and 1 x 120v dc.The 70 v dc is called E and the 120v dc A on a schematic FBT provided after a request (pretty similar to my unit with some different values - nothing identical exists in the company’s headquarter archives). I attach the schematic.Rail A is only used on the power amp circuit according to the schematic ( but I will note something at the end of my post).The 70v dc rail E is reduced to 40v to form the very important rail D, as well. Exactly like the schematic describes.E is only used for the bias oscillator and I can confirm the value on the circuit tap.D is used for the recording amp and I also confirm that value.But on the playback amps power tap (2 on the same pcb sharing the same tap) the D is reduced to 20v dc after a 4.7k 2watt resistor. Please keep a note of this resistor value for later. With these 20vdc the playback amps work ok.The tape echo seem to work with a signal to its in and things coming out of it. Tested with generator, oscilloscope and even amplifier. More like tape hiss and noise but still something goes on in there.I only use 2 input preamp circuits (out of the 6 the original unit has). The input preamps and the mixer power taps are connected together. Nothing more than 15-20v is meant to go in there since all the crucial components of those circuits (transistors, electro caps) have such max voltage ratings. I don’t think taking 40v dc directly from the D tap would be a good idea.So I took a tap from the 20v dc playback amps tap and connected it to the preamps and the mixer, but unfortunately the tap was reduced to 7v dc on all circuits involved (playback amps, input preamplifiers, mixer).I have to admit that i haven’t measured the recording and bias circuits on that state, just to check how their taps are influenced.With 7v dc the pre amps, work ok. I think the mixer as well since I have normal output. But there is nothing from the tape echo heads etc. So 7v dc is not adequate for it.The main question is if i could use a different simple external power supply of 9 vdc for the pre amps and the mixer and leave the tape echo alone on its 20vdc (and whatever goes on with the other parts of the echo circuit).I think a rating of 1A will be more than enough for such use.If i do so what should I take care of? 0v chassis common? Isolate the circuits and how?I mentioned before to take a note of a resistor value, because i would like to know if there is a way by changing that 4.7k 2 watt resistor (that reduces the 40v dc to 20vdc) to a different value, I would have adequate voltage for all of the circuits (12-15v for example) or the current consumption will be too much for the single rail?Before finishing I have to note that originally actually a tap from the big 120vdc rail A was used for powering the mixer-preamp circuit, but that was after the load of the power amp.So I guess an appropriate 8-10v dc voltage was available after that.But now the power amp is out of the equation, so 120v dc of course can’t be appropriate.Any advice, good and safe idea (very old unit, with big power amp rating and a quite huge main supply and transformer) or suggestion out of experience will be much appreciated.Sorry for the long post and thanks in advance for your time.

author: MathWizard
author_rank: Super Contributor
timestamp: July 15, 2025, 04:03:26 pm
content: I don't see any active voltage regulation, it looks like the loads and currents, drop the voltages as it goes. Then a tap further down, derived from A, would have provided 9V to the pre-amps and mixers, but those large power consuming circuits are missing? Power resistors and the various circuits/loads set the labeled voltages, so the "voltage rail value" depends a lot on the current through each power resistor.I would try to make a 9V rail from the 40V D rail. Have you used circuit simulators before ? That would be the quick way to see how much current and power the 9V rail would need, and how much it would change the 40V rail.Besides that way, I would find a small transformer, and make a rectifier circuit, and attach it's COM/GND to the GND of the pre-amp/mixer circuits.

author: Ian.M
author_rank: Super Contributor
timestamp: July 15, 2025, 05:02:24 pm
content: The circuit has no active regulation whatsoever, so if you remove a load on the E or D rails their voltage will rise. That means you need to either replace any module you remove with a big power resistor chosen to draw the same current at the nominal voltage to keep the rail loaded the same, or rebuild it with active regulation, or with a lower voltage transformer. Note that removing loads may also cause the 120V rail to rise a bit, and if the transistors in the output stage don't have enough Vceo rating, that may put them at risk. If you are simply not using the power amp, but have left it connected, that could be a problem. Assuming you have removed the power amp, I think I'd try and find another much smaller transformer suitable to give approx 40V after rectification for the D rail, remove the original transformer and fit an additional capacitor coupled bridge rectifier 'stacked' on top of its output, with capacitor values chosen to get 70V for the E rail.

author: tedsorvino
author_rank: Regular Contributor
timestamp: July 16, 2025, 12:31:56 am
content: Thanks for your replies and help. Additional small 8-9v supply it’s what I will try. Can’t remove the transformer since it has other ac taps useful for the unit, but not mentioned in the drawing (tape motors, voltage meter etc).

author: tedsorvino
author_rank: Regular Contributor
timestamp: July 18, 2025, 05:06:42 pm
content: For future reference. External linear supply, common 0v chassis grounded, parallel primary. Work well.The internal transformer should not be omitted. It’s used for the tape motor ac (along with other ac meter lines and the tape echo boards dc). The voltage selector increases and reduces the motor ac which increases and reduces the delay feedback (halo - self oscillation) speed.All good. Thanks for the help and input.