Quad 707 power amp mods

I've had this lump .....sat next to my gear not doing anything for a couple of months now.

So I decided to take a quick look inside.........!

I was not impressed by some of the parts but the actual build quality is very good.

So whilst I had the opertunity I hjad a look at the imput board for a start ....thr one thats soldered dirrectly to the rca sockets. I can't sat I've seen that done before !

The rca sockets are really cheap and nasty, so these were replaced by some half decent ptfe insulated ones.

The ground was a small tinned copper bent wire....this was lost ad replaced by silver wire.

Traceing the signal path on the input board .. it goes through a couple of resistors to the opamp. These were removed and replaced with appropriate Vishay's and the opamp fitted to a good quality socket...it was only a opa 2134 just what I had to hand.


The psu for this board uses so standard caps to I added a couple of silmic bypass caps to the main supply ones.

The signal goes through a pair of jumper pins ??? I could not work out why yet ....any ideas?

the signal is then tasken to the pair of power amp boards via one of the most auwful peices of coax I think I've ever seen and connected with spades.....spades !!!

Well this came out asap ....replaced with a twisted pair of 4n silver in ptfe sleeve.

I wanted to get the power amp boards out ...but was defeated buy the bolts ...you need an extra long screw driver thats thin.

Looking at the schematics ... there is a feed back loop with a opamp that is unusual.....TLc 271 cp........this is a biasable opamp pin 8 and 6 are connected the load on it determins the bias.

So the question is can this be swapped for a half decent jobie ?


I've to build a speakon adaptor for my current standard speaker cable connections before I can listen but that shouln't take long

 

The TLC opamp is a DC servo IIRC, not too critical as a result (unlike in the 405) but can be swapped out for something better. The 2134 will work well, since this is the input VAS a good opamp here can make all the difference. Pay attention to decoupling the opamp supplies though - these are zenered down from the main amp rails. Better regulation for the front end is a massive improvement.

PSU-related: add some beefy bypass caps on the poweramp boards by the output drivers (100-220uF, 63V+) - this will really enhance the midrange, esp into heavy loads.

Do a websearch for Bernd Ludwigs modification document for the 405/405-2 for more ideas, the ideas are common although the 606 and 707 have a slightly revised input stage which is much less noisy than the original 405.

 

cheers...............will look those out

 

What about the 1n4003.............would subing them for schottky 11dq10 work ? these 1n4003 are in the power supply to the trans. and across the legs of some of the trans.

rather than in the rectification of ac in the psu.

I'm familiar with there effects that schottky's have there but not this application.