5532 output from CD player

[AndrewT]

Hi all,
not sure if this is best forum.
How can I increase the output voltage from my Arcam 70.2 CD player above it's present 2Vac to about +20dbu (7.7Vac).
Preferably by altering the gain around the output chip rather than an additional output board.
I do not mind changing the chip to something more suitable but please avoid excessive cost ($20 chips are too expensive).

[Chris Daly]

Dear Andrew T
A bit of a loaded ( excuse the pun ) question
voltage as such for Cd players is usually specified
into a 600 ohm load. discussion of output voltage
must also have a current term as well.

Why do you need increased voltage ?

Ill discuss some of the boundarys around this
There are few power amps that are this insensitive ,
however to answer your query you could try beyond
the players opamp a LM1875 which is a audio ic
power amp,typically used to drive headphones.
Note however this is a ic power amp and is not an opamp.
Burr Brown have offered TO3 case power opamps.
The use of such devices alienates your system to function as
a conventional audio system- you may run into
real problems matching up audio power amps
which are specified closely to work with attenuation
from preamps or preferably passive pre amps
and to function well with conventional CD players.

You run real risk of overloading power amp inputs
by using such levels of output voltage. If you have
such a need - its quite likely there is a resistor soaking
up what little benefit there is to be gained from these
higher voltage levels- perhaps it may be wise to
check out what loading exists before going any further.

The existing opamp in your Cd player is typically
arranged as a current to voltage convertor -it usually
uses an inverting opamp stage. There are large differences
between models of opamp, and experimentation of
how they sound is well worth the effort. If you havent
done so already solder in a turned pin 8 pin socket
so you can interchange opamp models to assess
differences in sound. Although an old model the LF353
dual is exceptionally good.

A dual opamp is a compromise , whereas single opamps
offer better specification... depends on how fussy you are
Have a read of Ben Duncans texts and follow similar threads
in this forum to learn about opamp specifications and
recommendations, there are some really excellent
devices now available. Cheers / Chris

[AndrewT]

Hi Chris,
thanks for that considered reply and for the caution you advise.

I need more output to drive a Behringer DCX which in that thread recommends an input that feeds the digital circuits with a signal as close to digital max as possible without overloading. DCX spec says max input is +22dbu (9.75Vac). I then have -34db on the input into my amps.

I hope someone out there knows the 70.2 circuit and can post a mod to achieve my needs.

[EUVL]

I presume you have the Arcam (as you know the output opamp is NE5532) so it would take you half an hour to trace the output circuit around the opamp directly from the PCB.

The first thing I would try to figure out is the supply voltage to the opamp. Many players use the same supply for the DAC chip and the analogue circuits, which means that they have a low supply voltage (I have seen +/-9V to even +/-5V). If the voltage is lower than +/- 9V, you would have a lot more work to do before you can get 7V output swing.

I would also try to contact Arcam directly for schematics. I know at least one other UK manufacturer which provides them on request. Just quote your serial number and tell them why you need the schematics.

Otherwise not much help, I am afraid.


Patrick

[Fin]

OK - here is some info on this machine which might help in preventing you from destroying it.


1. It doesn't have an NE5532. It has two NE5534's.

2. The supply voltage to these opamps is derived from a nice discrete regulator and is +/- 12V.

3. The DAC uses a different power supply - (+/-5V and -15V) - from 7805/7905 and LM337.

4. I have the schematics if you want them - and Arcam also happily supply them for free.

5. Heading down your intended path will probably have an adverse effect on the beautiful sound of this CD player.

6. There must be another, less destructive, way to get the required input voltage for your amp.


Regards

[wa2ise]

If the op-amps' power supply is +/- 12V, you should be able to get them to do the 7.7VAC if this is a peak rating, not RMS. If it is RMS, the peaks would be 10.78V, which is likely too close to the supply rails for most op-amps to work correctly. But if it is 7.7V peak, all you should need to do is change the op-amp feedback resistor. Take whatever it currently is and multiply that value by 3.85.
Select whatever standard resistor value is closest to that result, and replace the old feedback resistor with it. If the board uses surface mount resistors, this will require some skill.

[AndrewT]

Thanks all,
If the output opamp is also the DAC then I will leave well alone.

The cd feeds a pre that has RCA (unbalanced) input and XLR (no gain) balanced output in // with record out (RCA &+4db) and in // with pre out (RCA&variable gain). Unfortunately it is entirely SMD.

The output opamp may simply be a line out conditioner that achieves good current ability and adequate voltage drive for normal domestic purposes. It worked admiably for me in this role since 1990 without modification. When I opened it up many years ago I recall seeing the two transformers and I'm fairly sure a 5532 dual adjacent to the output RCAs.

I could build an outboard balanced line driver with 8db of gain so the pre can drive the DCX but then all the other inputs will underdrive the DCX. As an experiment I wanted to do the very simple mod as above & check how things operated before even contemplating embarking on a much bigger project.

Next step obtain a schematic.
Arcamaniac, any chance of an Email attachment?
thanks again

[Fin]

Hi Andrew

Send me a private mail (through my contact details in the member's section) containing your email address - and I'll send you a copy.

Regards

[AndrewT]

Hi all,
thanks to Arcamaniac I now have the service manual with schematics.
The final opamp is not a gain stage nor line driver.
It is a 2 pole Butterworth filter with fixed gain to set the Q=1/sqrt2. and it runs on +-11v rails.
I'll need to find another easy way to increase my gain before my Xover.

[Chris Daly]

Dear Andrew T
Maybe a simple common emitter NPN self biasing stage
might suffice. ( BC547 or 2N2222 )

Use collector bias at IKohm for each volt of supply
then I megohm between the collector and base
ground the emitter and capacitively couple( easier ) or
transformer couple base input and collector output.

This should give quite a substantial gain increase
at low cost, and is pretty reliable circuit as well.
There are many other variations to try , but this one
is very easy to construct.

Hope this helps Cheers Chris