So I have built a relatively simple amplifier/receiver selector switch that allows the operator to select a 1 of 24 vintage amplifiers via a rotary cam switch. The device has a pair of Nixie tubes that indicate what amp is being selected at anyone time via a MOSFET board that is running IRF840s to switch from the selector switch to the RCA outputs. I added resistors to the MOSFET gates, 10kΩ (1/4W), (one per MOSFET gate to limit inrush current). There are pull-down Resistors: 12x 100kΩ (1/4W) (one per MOSFET gate to pull it LOW when not active). I am using a bridge rectifier with AC line voltage of 120VAC, change it to DC and I have added a 10uF 450 capacitor. This cap brings the line voltage from 120VAC to 170VDC and the cap will also steady the ripple. All of this, simply controls the Nixie tubes. I also added an Amphenol CL-110 thermistor in series with the capacitor to the positive terminal to protect things down the line from potential voltage spikes.

As a precaution, I put 13.6VDC to the cam selector switch rather than 170VDC. I figured the switch will last longer and the risk of shock is greatly reduced. I have two buck converters tied to the transformer driving the MOSFET gates and the pilot light(s). Right now I have 3 fuses, one for 120VAC, one for 120VAC to the 16VAC transformer and one for 170VDC for the Nixie tubes.

All of that being said, I needed a way to control the RCA inputs from the selected amps, so those input lines are controlled by the other two layers of the cam switch. They work as they should, but they DO NOT control any of the amplifier speaker OUTPUT signals. This is a problem when it comes to having more amps than speaker sets. So, I need to integrate another MOSFET board/s to control the selection from the Nixie tube MOSFET board to another board that will control the selected speaker outputs. Since amplified speaker signals cannot simply be shared on a common bus bar, I will need to come up with another MOSFET board. Here is the plan:

I will need 96 more MOSFETs to take the signal from the Nixie tube MOSFET board in order to run 24 unique amps/receivers. There will also be 4 pairs of speakers so they will need a bank of 8 relays to select the inputs and then change over to each set of speakers.

My issues are as follows:
1) So I will be using 96 MOSFETs, IRF3205, with high enough voltage and the ability to switch with a low input (13.6vdc) The boards will have to be stacked horizontally, 6 wide. This will be a lot in a small area, gonna take a little time.
An example of one IRF3205 MOSFET in the speaker system is:
- Speaker output #1 (e.g., Left+ of Receiver 01):
- Gate → Connect to both '0' gate and '1' gate lines from Nixie MOSFET board
- Drain → Connect to Left+ speaker input from Receiver 01
- Source → Connect to Left+ speaker selector output

2) There will have to be a large and crammed input board for all of the speaker signals from the amps and receivers. That board will have 24 sets of 4 pin connectors. Those are all going to be mounted on a 2.54mm perf board and then pass through the panel via existing holes. I will have to number the sets and use lots of colors of wire to ensure proper order and reducing confusion. They will wire up to each board in the above issue, based on colors. I think 6 unique colors will eliminate a lot of issues. I would like to use Molex edge connectors to make connections between the inputs and the boards as well as the interface with the Nixie board. Most Molex 2.54mm spaced edge connectors are rated rate around 3-4 amps, close the edge of what I feel comfortable with. I guess we will see what happens.

3) I don't love the above 4 pin perf board set up and I would much rather have something else. I also am thinking of using terminal strips on the back to connect to and from the MOSFET boards.

Could anyone suggest a better method? Could anyone make Gerber plans for this and I could send them out? Any suggestions, I am all ears.

 

Nixie tube MOSFET board

 

A video of the Nixie tubes in operation.

https://drive.google.com/file/d/1-ZZSxoUzWGR22MpWbU1f9uX-ZocAprYI/view

 

How are you switching audio signals with MOSFETs? Circuit please.

 

How are you switching audio signals with MOSFETs? Circuit please.

The speaker outputs? Or the amp RCA inputs?

 

The RCA outputs go through the cam switch via RCA plugs.
RCA output -> 22AWG shielded wire -> cam switch ->common line on cam switch -> RCA Input

 

Speaker outputs to the custom selector are planned to be like this.
Should the operator select amp "1", the Nixie tubes will display "01"

-Speaker MOSFET Gate → Connect to both '0' gate and '1' gate lines from Nixie MOSFET board
- Speaker MOSFET Drain → Connect to Left+ speaker input from Receiver 01
- Speaker MOSFET Source → Connect to Left+ speaker selector output

Signal	Connected To
Drain (Input Side)	Connected to amplifier’s L+ or R+ output
Drain (Output Side)	Connected to common speaker selector input (L+ or R+)
Source (both MOSFETs)	Tied together for each pair — floating, not grounded
Gate (both MOSFETs)	Tied together → driven by gate control from selector logic (13.6V through 330Ω + Zener + pull-down)

 

Complete MOSFET Gate Mapping Chart (Positions 01–24)
Each position activates 4 MOSFETs (L+, L−, R+, R−) using a shared gate control line.

| Selector Position | Receiver Name | Gate Signal Label | MOSFETs Activated |
|-------------------|----------------|--------------------|--------------------|
| 01 | Receiver 1 | GATE_01 | Q1–Q4 |
| 02 | Receiver 2 | GATE_02 | Q5–Q8 |
| 03 | Receiver 3 | GATE_03 | Q9–Q12 |
| 04 | Receiver 4 | GATE_04 | Q13–Q16 |
| 05 | Receiver 5 | GATE_05 | Q17–Q20 |
| 06 | Receiver 6 | GATE_06 | Q21–Q24 |
| 07 | Receiver 7 | GATE_07 | Q25–Q28 |
| 08 | Receiver 8 | GATE_08 | Q29–Q32 |
| 09 | Receiver 9 | GATE_09 | Q33–Q36 |
| 10 | Receiver 10 | GATE_10 | Q37–Q40 |
| 11 | Receiver 11 | GATE_11 | Q41–Q44 |
| 12 | Receiver 12 | GATE_12 | Q45–Q48 |
| 13 | Receiver 13 | GATE_13 | Q49–Q52 |
| 14 | Receiver 14 | GATE_14 | Q53–Q56 |
| 15 | Receiver 15 | GATE_15 | Q57–Q60 |
| 16 | Receiver 16 | GATE_16 | Q61–Q64 |
| 17 | Receiver 17 | GATE_17 | Q65–Q68 |
| 18 | Receiver 18 | GATE_18 | Q69–Q72 |
| 19 | Receiver 19 | GATE_19 | Q73–Q76 |
| 20 | Receiver 20 | GATE_20 | Q77–Q80 |
| 21 | Receiver 21 | GATE_21 | Q81–Q84 |
| 22 | Receiver 22 | GATE_22 | Q85–Q88 |
| 23 | Receiver 23 | GATE_23 | Q89–Q92 |
| 24 | Receiver 24 | GATE_24 | Q93–Q96 |

 

No comment you provide a schematic.

 

I have no way to do that.........I wish I did

 

Still not clear as to what device exactly is used to switch the outputs from the amps to the speakers.
Its' better to use relays for complete isolation, rather than MOSFETs.

 

Still not clear as to what device exactly is used to switch the outputs from the amps to the speakers.
Its' better to use relays for complete isolation, rather than MOSFETs.

AUDIO INPUT (Line-Level RCA) → Amplifier

Source	Path	Destination
CD, TV, AUX, etc. (RCA)	RCA left/right → cam switch deck 2 & 3	RCA jacks of selected amp
Selector Control	24-position cam switch (mechanical)	Selects 1 of 24 amps
Visual Feedback	Cam switch also controls Nixie digits	Nixie tubes show selected amp number

2. Nixie Tube Display System

Nixie Tube	Connection	Notes
Nixie Tube Anodes	170VDC via 10k–22kΩ resistors	Current-limiting
Nixie Cathodes	Connected to drains of 13 IRF480 MOSFETs	Digits 0–2 (right tube), 1–9 (left tube)
Nixie Gates	Driven by cam switch logic or decoder	Selectively enables cathodes

3. Nixie MOSFET Board → Speaker MOSFET Board (Gate Drive)

Source	Path	Destination
Cam switch logic / Nixie MOSFET gates	13.6V HIGH when selected	Drives speaker MOSFET gates (IRF3205s)
Gate lines	24 total	One gate drive per amp

4. Amplifier Speaker Outputs → MOSFET Output Switcher Board

Amp Output	Path	Destination
Amp L+, R+	To IRF3205 input drains	96 total input wires (48 channels × 2 MOSFETs)
IRF3205 Pair	Back-to-back, source tied	Allows AC pass-through when gate is HIGH
MOSFET Output	Shared line to speaker selector input	Only active amp passes signal
Speaker Ground (L–/R–)	Usually direct from amp	Or routed via common ground bus

5. Speaker Selector Output (L+/R+) → 8-Channel Relay Board

Input	Path	Destination
Selector output L+/R+	From shared MOSFET drain lines	To common input of relay board
Relay Board (8-channel)	4 relays for L+, 4 relays for R+	Output to 4 speaker pairs (A/B/C/D)
Controlled By	4-position speaker selector switch or logic	Activates 1 speaker pair at a time

6. Speakers

Signal	Path from Relay Board	Notes
L+ / R+	From active relay outputs	Routed to one speaker pair
L– / R–	From amp directly (or via shared GND)	Not switched unless full isolation needed

 

I got the pieces from the remains of a similar project about 26 years ago It was a very nice bunch of shielded cables and several nice connectors, and some hopeless electronic blobs.
I suggested building patch panels, with TRS jacks (Tip, Ring, Sleeve) for the stereo inputs and double T-S jacks for the two amplifier speaker outputs, and the available speaker pairs. That is because some amplifiers used bridged outputs, meaning there was no COMMON for the speaker connection.
Not a single bit of active electronics in the setup, and once all the connections were correct it worked very well. A Patch Panel will take up more space, possibly, but it will be much more reliable, and more fool-resistant.
I do not predict success with transistor switching of speaker outputs.

 

some amplifiers used bridged outputs, meaning there was no COMMON for the speaker connection.

Why I suggested relays for the switching.
If you ground a bridged output from an amp there's a good chance it will zap something in its output stage.

 

I got the pieces from the remains of a similar project about 26 years ago It was a very nice bunch of shielded cables and several nice connectors, and some hopeless electronic blobs.
I suggested building patch panels, with TRS jacks (Tip, Ring, Sleeve) for the stereo inputs and double T-S jacks for the two amplifier speaker outputs, and the available speaker pairs. That is because some amplifiers used bridged outputs, meaning there was no COMMON for the speaker connection.
Not a single bit of active electronics in the setup, and once all the connections were correct it worked very well. A Patch Panel will take up more space, possibly, but it will be much more reliable, and more fool-resistant.
I do not predict success with transistor switching of speaker outputs.

Got it. As it stands right now, I do not plan on sharing any grounds from the speakers or amps so that should make this in full isolation. The speaker MOSFET board basically talks to the Nixie board to determine which amp is selected for output. From there, the speaker signal will be carried via an 8 channel relay.

https://www.amazon.com/dp/B099MVQ5Q5?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1

 

Have you breadboarded the MOSFET switching circuit?

 

Have you breadboarded the MOSFET switching circuit?

I have not, I will and see if the current 'idea' to work. I am waiting to get the IRF3205s via the post office, stay tuned.

 

I have no way to do that.........I wish I did

There are over two dozen components on that board. Are you saying you wired up all of that with no schematic / wiring diagram / other - ?

It doesn't have to be pretty - a phone photo of a clear and readable cocktail napkin is fine.

ak

 

There are over two dozen components on that board. Are you saying you wired up all of that with no schematic / wiring diagram / other - ?

It doesn't have to be pretty - a phone photo of a clear and readable cocktail napkin is fine.

ak

I wired the first one with no schematic. Just replicated where the Gates, Drains and Sources should go. The second MOSFET board, no schematic. There will be 96 total MOSFETs, 24 selections with 4 lines into the board (R-, R+, L-, R-). Once one works, it is basically a series of replicated connections.

 

There are over two dozen components on that board. Are you saying you wired up all of that with no schematic / wiring diagram / other - ?

It doesn't have to be pretty - a phone photo of a clear and readable cocktail napkin is fine.

ak

This was really helpful when it came to wiring things up. I used the N-channel arrangement.
https://www.instructables.com/Transistor-Basics-MOSFETs/