insert USB soundcard in old Pioneer amp, help needed

will the circuit be ok without the resistors from the DIN?

  • yes

    Votes: 0 0.0%
  • no

    Votes: 0 0.0%
  • see my comments

    Votes: 0 0.0%
  • a professional may be needed now

    Votes: 0 0.0%

  • Total voters

Thread Starter


Joined Sep 6, 2010

I bought an amp on ebay (Pioneer SA-700) and found that the input terminals were too close together for my leads, so it got me thinking about how I would fix it. I decided to installing a small USB sound card in the unit in the place of the DIN-5 port (cassette deck in/out).

I have modified the sound card to a type B USB port. The next thing was to remove the DIN port and wire up the sound card's audio in/out to the tape in/out ports. Removing the DIN port was a bit difficult due to space and so I ended up just hacking it out. Being a long time since I've done anything like this, I didnt think about the way the DIN was connected having any relevance to the rest of the unit. In my mind the DIN was just daughtered off the other tape deck ports (MON and REC). Now I have looked at the circuit diagram I am not so sure.
I have uploaded the diagram to a simple webpage to be found here, nothing fancy I'm afraid. There are also 3 photos of the area affected.

I have removed the DIN-5 port and the 6 resistors attached, wondering now whether removing the resistors was a bad idea, I thought it would be isolated and the resistors just related to the DIN operation, now I'm not so sure.
I did also blow a fuse last night, I dont know if I shorted something while testing, but may be related.

Any ideas on how to proceed? do I attempt to rebuild the circuit related to the DIN 5 port with the resistors? Is there an easier way? the unit is quite tight and hard to get in close, so I have used long green leads as a result so I can bring my work out of the box.

Unrelated, I am also upgrading the speaker ports to higher quality ones as I couldnt find the plug they used to use.

After all this is done and working, I am thinking about inserting a TOSLINK unit (Jaycar has kits available) and maybe even an ethernet media player... although I may just keep it outside the box due to heat loads.

many thanks


site link

btw, love how the old amps include full documentation, they were obviously proud of their equipment back in the day!


Joined Sep 4, 2010
mmmm ...
You may want to look at some audio line driving theory before cutting into the thing any more.

Don’t get me wrong I am always up for a little experimentation, and I am not criticising either its just that in this case a small amount of background knowledge would go a long way.

Firstly what I am about to say is about 50 pages short of a ‘real’ explanation of what you are dealing with.

All your IO , will have been designed to drive or accept a specific maximum Peak to Peak signal over a line with a specific characteristic impedance. It is likely that all the IO is unbalanced as opposed to balanced lines. (CoAx)

In practice ‘whilst tinkering’ and again this is an oversimplification, you should make any extensions to the board in coax so they are shielded or you will get hum. RE your green wires.

Outputs will usually have a series resistor close to the plug and inputs will have a resistor from your line, central conductor, to ground and probably a series resistor as well to ‘scale’ the signal to the input stage.

If you look up some audio amp chips and examine the simple application circuits you will see what I mean.

Line level specs on modern equipment are usually extremely forgiving, older kit just wont work that way.

Do a google search and have a look at the differences between Phono & line also note that studio and domestic equipment is / was different RE Line.

You certainly can play with this stuff and relative easily modify an amps IO capabilities, within limits and dependent on what you can get at after understanding its function.

Lastly, if I had a pound for everything I have blown up or failed to get working I would be very rich
But using the basic info above to go and find what i really needed to know is how I stopped blowing things up, so regally, and was able to do just the sorts of things you are talking about.
Have fun
Last edited:


Joined May 11, 2009
Can you explain in a few words trying to do? Are you trying to embed a USB sound card into your amplifier, so you can have digital link from your PC to your power amp. And why are soldering cables on the inside of the amp like you have done.

Thread Starter


Joined Sep 6, 2010
yes I have inserted a USB soundcard inside the amp, this is to make it simpler to connect PCs to it (the old connections on the back are way too close together for most cables I own, so instead of pushing them apart I thought I would insert the soundcard so only one cable would be required to connect a PC to it (a USB cable).

Soldering was done as otherwise there would be a mess of cables on the outside of the box. I am about to upload new photos on the site to show how I have done it.

As it is the sound quality seems ok, I am a bit worried about the resistors I removed, but with the fuse replaced it all seems ok. Does anyone know if DIN-5 connections required signal modification? I wonder if the resistors were for that specifically.... oh well, it's all fun at the end of the day, and it seems ok so far.

Thread Starter


Joined Sep 6, 2010
I received a call from my local radio station (2MMM) today advising I have won a digital radio tuner, turns out I may have to do something about the input connections now anyway! Not knowing what unit is I have won, I wont make plans yet, but if it has a TOSLINK connection I may install a decoder and optical connection