Referring to the #35 schematic, why are there Zobel networks on the U4 and U5 outputs?

Also, the PWM square waves on the 8403's L+ and L- outputs are 180 degrees out of phase with each other. This means that as the R7 headset level pot is rotated from pin 1 to pin 3, the audio in the headphones will go from full volume at the CCW end (pin 1), through half volume, to zero volume in the middle when the two phases cancel out), through the other half volume, to full volume at the CW end (pin 3).

ak

 

One additional thing I see looking at that circuit schematic drawing is in the power supply area with the LM7806 voltage regulator. It is a THREE TERMINAL device, with the negative side of the input and the negative side of the output both being the same terminal. So there is no isolation at all for the common supply negative side. Normally that is not an issue, but I have not looked at the specifications of that PAM 3403 device. In addition, the 0.1 mFd capacitors that are specified at the input and output of the 7806 are not shown. My experience is that those small capacitors close to the actual regulator are required to avoid oscillations that will cause interesting problems.

 

We know nothing about the 7805 module, but many of the ones I've seen on ebay have the input and output returns directly connected together with a fat trace on the pc board. If that is the case with this module, then the +5 V and the +/-15 V share a common GND, which is correct for this circuit.

I am curious about the 987 uF capacitor on the output.

ak

 

So is the speaker amplifier Left input pin. Pin L on the amp module is based on the rotary selector which has three possible outputs. Simply tap Pin L.

I figure there'd have been a sufficient voltage drop from the 33171 that would require the 8403 to boost them. Am I mistaken?

 

Referring to the #35 schematic, why are there Zobel networks on the U4 and U5 outputs?

Also, the PWM square waves on the 8403's L+ and L- outputs are 180 degrees out of phase with each other. This means that as the R7 headset level pot is rotated from pin 1 to pin 3, the audio in the headphones will go from full volume at the CCW end (pin 1), through half volume, to zero volume in the middle when the two phases cancel out), through the other half volume, to full volume at the CW end (pin 3).

ak

Initially it probably came from a schematic I saw online. I think it was also suggested in a previous thread/discussion concerning high-pass/low-pass.
Do you think it is unnecessary?

As for the 8404 I didn't realize that it would produce a square wave output. I've attached a schematic i found online to convert. If you think it would do the trick could you suggest values for the components?

 

We know nothing about the 7805 module, but many of the ones I've seen on ebay have the input and output returns directly connected together with a fat trace on the pc board. If that is the case with this module, then the +5 V and the +/-15 V share a common GND, which is correct for this circuit.

I am curious about the 987 uF capacitor on the output.

ak

i contacted a seller of something similar who wrote: "If a generic rectified power supply is used, be sure to add a 1000uf 10v capacitor on the power input to avoid interference." I generally follow suggestions that make sense or don't defy logic.

 

i contacted a seller of something similar who wrote: "If a generic rectified power supply is used, be sure to add a 1000uf 10v capacitor on the power input to avoid interference." I generally follow suggestions that make sense or don't defy logic.

the item is at: https://www.aliexpress.us/item/3256...t_main.11.21ef1802KYhV0P&gatewayAdapt=glo2usa
i had actually not yet opened the package. but there are no thick wires.
i tested it with a 12V battery and it put out exactly 5V with no fluctuation. If I can assumed it's well regulated then i guess i don't need that 987uF cap.

 

One additional thing I see looking at that circuit schematic drawing is in the power supply area with the LM7806 voltage regulator. It is a THREE TERMINAL device, with the negative side of the input and the negative side of the output both being the same terminal. So there is no isolation at all for the common supply negative side. Normally that is not an issue, but I have not looked at the specifications of that PAM 3403 device. In addition, the 0.1 mFd capacitors that are specified at the input and output of the 7806 are not shown. My experience is that those small capacitors close to the actual regulator are required to avoid oscillations that will cause interesting problems.

the item is at: https://www.aliexpress.us/item/3256...t_main.11.21ef1802KYhV0P&gatewayAdapt=glo2usa
it's a module USING a 7806. the module has a dual-post input (for ~15V) and a dual-post for the 5V output. i just tested it with a 12V battery and the output was a non-fluctuating 5 volts. would you still recommend that i include a 0.1 uF cap at both ends.
the 8403 (https://www.aliexpress.us/item/3256...t_main.17.40821802oU5dYa&gatewayAdapt=glo2usa) has L,R and GND posts. But you raise an interesting point: do the plus and minus signs for the 5VDC source connection mean +5 and ground or +5 and -5? The 7805 has +5 and GND connections only.

 

That module has a bridge rectifier at its input, so it can run off of low-voltage AC or DC.

BUT

The bridge means that the input return and output ground are NOT connected. Thus, the schematic is incorrect. The purple 5V return *must* be connected to the blue GND.

This can be corrected with a jumper between the input and output terminal blocks, shorting out one of the diodes in the bridge.

ak

 

That module has a bridge rectifier at its input, so it can run off of low-voltage AC or DC.

BUT

The bridge means that the input return and output ground are NOT connected. Thus, the schematic is incorrect. The purple 5V return *must* be connected to the blue GND.

This can be corrected with a jumper between the input and output terminal blocks, shorting out one of the diodes in the bridge.

ak

thank you . . . so I can just attach the purple to chasis ground, no?

but what about #45, the square wave filter?

 

That module has a bridge rectifier at its input, so it can run off of low-voltage AC or DC.

BUT

The bridge means that the input return and output ground are NOT connected. Thus, the schematic is incorrect. The purple 5V return *must* be connected to the blue GND.

This can be corrected with a jumper between the input and output terminal blocks, shorting out one of the diodes in the bridge.

ak

OK, it seems that I am in error!! I made the evidently incorrect guess that a "7805 module" was actually an LM7805 five volt regulator. Why would any rational being use the same name as a product tat has been around and well used for at least 30 years??
And the bad news then is that for any bridge input stage device, neither input connection should be made common with either output connection.

 

I made the evidently incorrect guess that a "7805 module" was actually an LM7805 five volt regulator.

Close. It is a pre-assembled module with an LM7805 in the middle of it. It has a bridge on the input side and both input and output capacitors.

The 7805 is clearly visible in the photographs. Plus, I found a photo of the underside of the board, and the traces from the input connector to the bridge to the regulator are correct.

ak

 

That module has a bridge rectifier at its input, so it can run off of low-voltage AC or DC.

BUT

The bridge means that the input return and output ground are NOT connected. Thus, the schematic is incorrect. The purple 5V return *must* be connected to the blue GND.

This can be corrected with a jumper between the input and output terminal blocks, shorting out one of the diodes in the bridge.

ak

couldn't i just chassis ground the negative (black-purple) side of the 5V output?

OK, it seems that I am in error!! I made the evidently incorrect guess that a "7805 module" was actually an LM7805 five volt regulator. Why would any rational being use the same name as a product tat has been around and well used for at least 30 years??
And the bad news then is that for any bridge input stage device, neither input connection should be made common with either output connection.

 

couldn't i just chassis ground the negative (black-purple) side of the 5V output?

" for any bridge input stage device, neither input connection should be made common with either output connection."
Please point me where I've goofed in the attached.

 

Sorry that I post without reading the whole thread, I just want to add, there is a manufacturer for audio transformers.. OEP.. they have specialized small transformers for signal level change.. they have ratios like 1:1.4, or 1:3.something .. ratios that are exact mic->line level or vice versa. Just saying if you want to check them. I have used them plenty, you just need to add some resistors and you have level changer.

 

So the TS can save both money and prevent grief by avoiding that "7806" module and simply using an LM7806 regulator IC device.

AND It is not clear just what the actual intended function of the rather complex circuit is intended to be, so a statement of the specific design goals could possibly lead to a number of beneficial suggestions.

 

And the bad news then is that for any bridge input stage device, neither input connection should be made common with either output connection.

With an AC input, maybe. BUT in this case, common-grounding the input and output reduces the bridge to a single series diode. The 15 V input is more than enough headroom to cover that voltage drop.

Please point me where I've goofed in the attached.

In this area, I think you're fine. Reduce C13 to 10 uF, add a 0.1 uF ceramic cap, and make sure they are located as close as possible to the 8403 power pins.

ak

 

So the TS can save both money and prevent grief by avoiding that "7806" module and simply using an LM7806 regulator IC device.

Have to disagree. With the bare IC you have to purchase a heatsink, at least two capacitors, perf board, and connectors. And then assemble and test everything.

All of that for 95 cents - ? Even with shipping (ebay is cheaper), not even close.

ak

 

Sorry that I post without reading the whole thread, I just want to add, there is a manufacturer for audio transformers.. OEP.. they have specialized small transformers for signal level change.. they have ratios like 1:1.4, or 1:3.something .. ratios that are exact mic->line level or vice versa. Just saying if you want to check them. I have used them plenty, you just need to add some resistors and you have level changer.

thank you

 

So the TS can save both money and prevent grief by avoiding that "7806" module and simply using an LM7806 regulator IC device.

AND It is not clear just what the actual intended function of the rather complex circuit is intended to be, so a statement of the specific design goals could possibly lead to a number of beneficial suggestions.

What is "TS?"