I have a Quad 33 which was modified probably nearly 30 years ago now (12v battery power supply, new rear panel with phono sockets, new capacitors and many resistors throughout, apart from tone control circuit which I never use) I have noticed that there is a distinct lack of low bass using the disc input now. I am using a Shure V15 III through the disc adapter board, which was modified using Vishay resistors and new film capacitors to provide (apparently) optimal loading. The disc pre amplifier boards use Elna electrolytics which appear to be still within spec according to my DMM tester. Other inputs still sound fine as far as low bass is concerned, and I think that they all pass through the Tape board, so I am thinking that the problem may be in the Disc pre amplifier board. Could it be the electrolytic capacitors? It would be easy and cheap to replace these with modern low ESR types from Cricklewood Electronics, but I would be interested to hear any opinions/advice.
I also still have the original printed guide to modifying the 33 and 303 by Russ Andrews, which I used all those years ago, and I can scan and post them if anyone is interested.
I also still have the original printed guide to modifying the 33 and 303 by Russ Andrews, which I used all those years ago, and I can scan and post them if anyone is interested.
It could well be an electrolytic but it would be odd for both channels to be the same. As ever, the ideal scenario is to actually look at the response, perhaps using a signal generator and a simple passive Inverse RIAA network at the input. You could then see if the response was even or not.
Ultimately there is nothing component wise other than caps that can affect the LF response. Last time I had such a fault was on of all things a VHS recorder. An electrolytic in the audio path had 'lost goodness'.
Ultimately there is nothing component wise other than caps that can affect the LF response. Last time I had such a fault was on of all things a VHS recorder. An electrolytic in the audio path had 'lost goodness'.
More likely the problem is on the Disc Adapter board, the one that pulls out at the rear. There are two EQ capacitors per channel, the larger of which should be 29nF, or in some cases 22nF+6.8nF, and the other one is 0.01uF. These are C101/2/3/4. Check them. Not common for them to go out of spec, but it has happened.
Not so. The resistances in the RIAA network contribute to the time constants.
There Is a phono eq board that has several.settings depending on how it is installed in the 33. You can try changing its position.
Maybe an error in the renovation. All the caps on the adapter board should be checked.
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The capacitors on the Disc adapter board are Kimber Kaps for the 29nF and .01uF, polystyrene for the others, and Vishay resistors throughout. I'll lift them and test them individually although they seem to test ok on the board. I only have a DMM for testing so it may not be the best test, and I don't have a signal generator. The Tape Adapter board has a few changes to the standard capacitor values in order to give better LF response (I'm told) and CD & computer as source through DAC sound fine for bass. I will also try another turntable and cartridge combination just in case. The Disc Adapter board is meant to provide 47k and 330pF for the Shure cartridge, but I've never noticed a difference with other TTS in the past.
Thank you!
Thank you!
That's too much. The capacitor should be 180pF or so, but this only affects HF response, not LF.
Check the 8k2 resistors on the Adapter board, and the 120k resistors on the Phono Amplifier board.
This is a bit of an unknown, however I'll post it for interest. Don't feel you have to do this 🙂
Attached is an frequency sweep from 20Hz to 20Khz with inverse RIAA equalisation applied. Its an MP3 track and so will play/should play on anything. If this is attenuated with a resistive divider to drop the level it should be possible to apply it to a phono stage input and get a flat response out.
It would also be possible to create spot tones of say 40Hz, 80Hz, 160Hz etc and do the same and also create non equalised versions as well. You could then compare output levels using a meter on AC of the amp and check you see the same level for line vs phono.
Zipped file contains 60 second test track.
Audacity is free and it takes moments to create these.
Attached is an frequency sweep from 20Hz to 20Khz with inverse RIAA equalisation applied. Its an MP3 track and so will play/should play on anything. If this is attenuated with a resistive divider to drop the level it should be possible to apply it to a phono stage input and get a flat response out.
It would also be possible to create spot tones of say 40Hz, 80Hz, 160Hz etc and do the same and also create non equalised versions as well. You could then compare output levels using a meter on AC of the amp and check you see the same level for line vs phono.
Zipped file contains 60 second test track.
Audacity is free and it takes moments to create these.
The easy way to test RIAA compliance is to take the levels at 1kHz, 20Hz, and 20kHz. If the level at 1kHz is 0dB, the response at 20Hz should be +19.2dB, and the response at 20kHz should be -19.2dB. If these three points are accurate, so is the whole curve. In practice you will get both numbers being a couple of dB lower (i.e. closer to zero), but a major response anomaly such as you are hearing will be much larger.
ok - for the 330pF in parallel with 47k I just followed the recommendation from Quad which came on a printed update with the original instruction booklet. The 82k resistors are spot on, as are all the other Vishays on the Disc Adapter board. All the polystyrenes are well within limits and pretty closely matched. The Kimber Kaps, however, measure .023uF and .0086uF instead of .029uF and .01uF. The .33uF Kimber Kaps C305 & C306 on the Phono Amplifier board measure .029uF and all the Vishay resistors are pretty much spot on, certainly well within limits, although there were a few which I had incorrectly installed (embarrassingly, although there are a couple of misprints on the Quad service supplement illustrations) Maybe the Kimber Kaps are just not very good and I should replace them anyway?
Here are the diagrams/lists I followed. The other change is that C313 on the phono board is 470uF instead of 100uF, as recommended by Dada - supposed to help with LF response, but maybe it's wrong? Still no bass.....
And the .029uF that should be .33uF. This is a huge error.
That 470uF promotes better bass is a myth unfortunately propagated by an ex-colleague.
That 470uF promotes better bass is a myth unfortunately propagated by an ex-colleague.
I'm still puzzled. I tried the reverse RIAA file and it shows what I hear - no bass, even using another TT/cartridge. I have changed the capacitors on the Disc Adapter board, and they now measure around .3uF instead of .29uF. I also changed the electrolytics on the Phono Board. The Quad manual specifies C300, C303, CC307, C308 as 25uF, C311, C312 as 50uF. The Russ Andrews spec which I originally followed states 20uF for the 25uF caps, and 50uF for the 50uF, these values being made up using 10uF and 100uF in parallel/series respectively. I have used 22uF and 47uF, but I see that Dada recommends 47uF for all of them. What difference would this make, and could anything else account for the problem? All resistors measure fine. At this stage I'm almost tempted to give up and use another phono stage......
No difference at al within these limits. The capacitors that seriously affect bass are the ones I have already mentioned.
EJP
EJP