This Blog is about revising/upgrading the vintage Quad 33 to 909 amplifiers and the Revox taperecorders.
Audio electronics and High End Hifi are also absolutely "on-topic" here.
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a question on the forum about a remote controlled
volume control, the intention was to place it between
a Quad 33 and 303. At that stage we could not help, so
we advised to look at a kit or a complete unit based on
a PGA2311 chip. On eBay there are plenty
of companies who can deliver such a box or kit
in various forms. I also bought such a system as a kit,
more or less a kit, the electronic modules are already
assembled. After a few weeks the kit was delivered. I had to ad
a transformer and some mechanical fixings to get the
kit operational. Some first remarks, there
was no manual provided, so this kit is not for
absolute beginners. Secondly there was no provisioning for
a fuse holder. I measured the
unit, it performed well. It sounded just allright. I found a schematic on the web. I checked the
schematic with the unit under test, the
schematic matched my unit. I also downloaded the datasheet
of the PGA2311. The first problem was there already, the unit
was DC in and DC out on the signal in and
outputs, for my own equipment I find AC in and DC
out acceptable, but for a commercial and/or
professional product, AC in and AC out is the
norm. Also they added 10K resistors in parallel to the
input, probably they thought the input was connected via a
buffer stage, but the input of the PGA2311 behaves like
a 10K resistor already.
In this way they effectively reduced the
input impedance to a lowish 5K. This (can) give serious frequency
response errors when driving from an AC coupled
high impedance source preamp. According to the datasheet
the PGA2311 input must be driven from a low impedance
source, our friends put 220 Ohm in series with the
input, with higher distortion as a result. The 220
Ohm resistors make sense, if you
complement them with small caps to form a RF/EMC
filter, but without those, you only add
distortion. Also they 'forgot'
the decoupling capacitors which must be placed as
close as possible to all the power supply pins of the PGA2311.
In the output they put 330 Ohm resistors, in it self
a good thing for stability when driving real cables
and inputs of power amps, but 150 Ohm is
a better value or compromise if you wish.
So I replaced the 10K
input resistors with 220K bleeder resistors. The 220 Ohm
series resistors in the input circuit where replaced by 220uF bipolar
capacitors, this might look as overkill, but to keep
the distortion of the capacitors low, you have to reduce
the AC voltage across the cap. And I put all the decoupling capacitors
in place, according to the datasheet. The resistors in the
output where replaced by 150 Ohm. The unit measured
and performed ok, this unit was programmed for 1dB
steps, which worked very good. Also you could
select three sources. Which makes it a preamp.
The only thing which wasn't there was a balance
A general remark about the PGA2311
chip, it will only give very
low distortion when it is used with high level
signals, high levels compared with the Quad standard of 500mV for their power amp inputs and
pre amp outputs.
So to get minimum distortion,
the levels between the Quad equipment must be
raised. So the output of the preamp must increase, but
more importantly the input sensitivity of the power amp
must be set lower. The PGA2311 based unit performs
at his best when the 0dB setting means full living room
Then the last check. I bought a PGA2311
PA chip from our regular supplier and placed it in
the unit...........The distortion and noise went slightly up! So the chip supplied
was a real PGA2311 PA. Very good indeed!
Very good casing and mechanical work.
Electronic modules were perfectly
Ad some form of manual, real or
Follow the advice in the
datasheet from the manufacturer about decoupling.
Redesign the input and output circuit
to make it a real world preamp.
So with some
work, you can improve this kit or
unit from good to very good!
Ps, the display is blue, I changed it with a red filter!