QUAD 33 phono cards

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#41
jkojic said:
does the line ins sounding amazing mean that power supply filter caps can be discounted from the thin sound- also on ebay Ferndale replacement boards - any opinions?
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Discounted... almost certainly yes but that doesn't mean they wouldn't benefit from replacement.

Without detailed measurement its all guesswork but all the electrolytics in the signal path (and that means those in feedback networks as well) are suspect.

Without measurement its all subjective. Thin sounding is a phrase often applied to some solid state amps even though their response is flat. If yours sounds like an amp with the bass control turned to minimum (on phono only as you describe) then caps in the signal path are really the only suspects.
 
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#42
I totally agree with Mooly about caps... I would suggest in the absence of measuring equipment to put a vinyl for disc or a source that you know there's a lot of bass. Even with center setting of bass control you should feel it. But also make sure the amp is running good, since they are a pair!
 
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#43
When I look at QUAD 33 I see similarities with a newer design (1996) made by m.Douglas Self. I joined a PDF and you'll see that both have variable SLOPE in filter except that the precision preamp of m.Self has that slope control on both bass and treble. My goal here is not to make advertising of m.Self's design. There's already a selling thread about his preamp here on DIY since 2015. Since QUAD 33 is now rare to find, precision preamp is a good alternative.
OK I will now stick to the QUAD 33!
 

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#45
I'm not a fan of measuring caps tbh, they are cheap and cheerful enough to replace. Most parts whether caps or resistors or semiconductors need to be removed or isolated from the circuit to get a true result.

Although it is true that caps often get replaced when there is no need, the Quad definitely falls into the other category of 'replace'. The handful of electrolytics on the phono boards can be swapped in minutes.

Ideally you should really locate the fault by test and measurement (scope and generator) if only for the satisfaction of finding the issue.
 
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#47
If you replace the caps then make a note of which way around they go and if needed take pictures. Board markings aren't always clear and occasionally can be wrong and so fit like for like and the same way around as what comes out.
 
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#49
marc2009 said:
When I look at QUAD 33 I see similarities with a newer design (1996) made by m.Douglas Self. I joined a PDF and you'll see that both have variable SLOPE in filter except that the precision preamp of m.Self has that slope control on both bass and treble.
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The Precision Preamp '96 has variable frequency first-order tone controls. Not variable slope. There is no similarlty.
 
#50
Thanks ejp for your comments... it forces me to revisit QUAD 33 web sites and understand goals behind design. I give reference address of web sites visited. Schematic of QUAD 33 was taken from web site and in fact TR200-201 doesn't exist in the one I re-looked up. I join PDF redrawing of chematic and web address for reference of viewing! PDF is now showing TR400 as input buffer input and TR402 as output buffer with their collector tied to +12V, leaving TR401 as Class A amplifier. No comments about DISC input. Just change electrolitic caps.
Now the Slopes... in the the QUAD 33 it's finally an extra low pass filter at the output of preamp with selectable frequency references of 5kHz, 7 kHz or 10 kHz and SLOPE control to fine tune, like to vary Q of filter. The goal as I read article was to remove "POPCORN'' from DISC vinyl program... no need to use it in other selections.
And precision preamp does move the SLOPE of the bass and treble. Actually on the bass it move the corner of 1st order bass filter from 1 kHz to 10 kHz and the corner of the 1st order high pass filter from 1kHz to 10 kHz. Literature is very accessible from m.Doug Self's books and articles.
So to do like QUAD did for precision preamp, we would have to add a selectable frequency low pass filter with adjustable SLOPE control at output of preamp....
Enloy references and feel free to react!
 

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#51
It was to late to edit my wriiting... about SLOPE. Precision preamp doesn't move SLOPE but the corner frequency said in post before (bass = 100 Hz to 1 kHz and treble = 1 kHz to 10 kHz). Slope will stay the same.
SLOPE control in the QUAD 33 doesn't change the corner frequency but the dB attenuation over that corner frequency. Those corners are selectable between 5 kHz, 7 kHz and 10 kHz.
Sorry for the but-in informations... the rest stays the same!
 
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