Quad 33 attenuation of output - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

stevehertz · 23rd Oct 2022, 5:13 pm

Anyway, you got there in the end and that's what matters. Glad for you.

mikes08 · 24th Oct 2022, 7:40 am

Thanks all.
Basic error and plonker springs to my mind but nothing blew up

...lol and it is all good fun learning the ins and outs of these amps though i am more familiar with the inside of the 303. I listened to it yesterday and that amount of attenuation seems about right.
The basic question still remains for me to investigate still - why two pre amps built to the same spec have different levels of output

...lol

Mooly · 24th Oct 2022, 11:46 am

You really need to have an accurate way of measuring signal levels to track that one down. I think you mentioned a DVM earlier... you would have to try it, initially looking at the input voltage at the input socket and confirming the DVM accurately (or at least repeatably) measure the level OK.

Any test tone would need to be a sine and of around 400Hz to be within guaranteed range of most DVM's

A level of say 500mv rms ideally should show to a few decimal points such as 0.500 Absolute accuracy doesn't matter but repeatability does. Also we do not know the input impedance of the DVM (a scope is high at 1meg or 10meg) and so we would need to measure within the amp at known low output impedance points.

A test tone is super easy to create in seconds with something like Audacity and can be saved and played as an MP3/WAV/WMA etc or burned to a CDR.

Ted Kendall · 24th Oct 2022, 12:22 pm

Quote:

Originally Posted by mikes08

Thanks all.
Basic error and plonker springs to my mind but nothing blew up

...lol and it is all good fun learning the ins and outs of these amps though i am more familiar with the inside of the 303. I listened to it yesterday and that amount of attenuation seems about right.
The basic question still remains for me to investigate still - why two pre amps built to the same spec have different levels of output

...lol

Perhaps the other 33 has the resistor mod already?

mikes08 · 25th Oct 2022, 8:02 am

[/QUOTE]Perhaps the other 33 has the resistor mod already?[/QUOTE]

Hi Ted - no - checked that first thing yesterday

Thanks Mooly - I did check the output with meter set at 500mv with the pre amp powered up but no signal. I am sure the reading on the meter was something like .005 but that may be telling me nothing though i will check against the 33 i was using and see how that compares.
Other than that it is back to comparing the plug in boards - one against the other to see if anything is amiss.
All academic really as they are both working fine but just curious.

Mooly · 25th Oct 2022, 11:07 am

You should really see 0.0000 volts with no signal. A DVM should be far to insensitive to show any noise or hum present.

With a signal we apply a known level to the input (I'd say around 500mv rms a good initial choice) and then we look at levels from input through to output to see where any differences might be.

I can post an MP3 file of test signal/s here if you want to try. You would need a suitable device to play it on and to be able to feed that line level signal to the input of the amp.

23rd Oct 2022, 5:13 pm	#21
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,839	Re: Quad 33 attenuation of output Anyway, you got there in the end and that's what matters. Glad for you. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

24th Oct 2022, 7:40 am	#22
mikes08 Pentode Join Date: Dec 2021 Location: Canterbury, Kent, UK. Posts: 136	Re: Quad 33 attenuation of output Thanks all. Basic error and plonker springs to my mind but nothing blew up ...lol and it is all good fun learning the ins and outs of these amps though i am more familiar with the inside of the 303. I listened to it yesterday and that amount of attenuation seems about right. The basic question still remains for me to investigate still - why two pre amps built to the same spec have different levels of output ...lol

24th Oct 2022, 11:46 am	#23
Mooly Octode Join Date: May 2011 Location: Lancashire, UK. Posts: 1,351	Re: Quad 33 attenuation of output You really need to have an accurate way of measuring signal levels to track that one down. I think you mentioned a DVM earlier... you would have to try it, initially looking at the input voltage at the input socket and confirming the DVM accurately (or at least repeatably) measure the level OK. Any test tone would need to be a sine and of around 400Hz to be within guaranteed range of most DVM's A level of say 500mv rms ideally should show to a few decimal points such as 0.500 Absolute accuracy doesn't matter but repeatability does. Also we do not know the input impedance of the DVM (a scope is high at 1meg or 10meg) and so we would need to measure within the amp at known low output impedance points. A test tone is super easy to create in seconds with something like Audacity and can be saved and played as an MP3/WAV/WMA etc or burned to a CDR.

25th Oct 2022, 8:02 am	#25
mikes08 Pentode Join Date: Dec 2021 Location: Canterbury, Kent, UK. Posts: 136	Re: Quad 33 attenuation of output [/QUOTE]Perhaps the other 33 has the resistor mod already?[/QUOTE] Hi Ted - no - checked that first thing yesterday Thanks Mooly - I did check the output with meter set at 500mv with the pre amp powered up but no signal. I am sure the reading on the meter was something like .005 but that may be telling me nothing though i will check against the 33 i was using and see how that compares. Other than that it is back to comparing the plug in boards - one against the other to see if anything is amiss. All academic really as they are both working fine but just curious.

25th Oct 2022, 11:07 am	#26
Mooly Octode Join Date: May 2011 Location: Lancashire, UK. Posts: 1,351	Re: Quad 33 attenuation of output You should really see 0.0000 volts with no signal. A DVM should be far to insensitive to show any noise or hum present. With a signal we apply a known level to the input (I'd say around 500mv rms a good initial choice) and then we look at levels from input through to output to see where any differences might be. I can post an MP3 file of test signal/s here if you want to try. You would need a suitable device to play it on and to be able to feed that line level signal to the input of the amp.