Long story short, got a Quad 34 and a Quad 405-2 from a friend. Checking for faults (none found except leaky electrolytes), recapped and put everything together again. Everything seems to work just great but I find the disc input too sensitive. The disc module seems to be the MM module, marked; 3mV, 47K/220p. I checked all components against the 3mV schema and everything checks out (except E5270 in the schema being BC413C on the board). I hooked it up to a Technics SL-23 with its National Panasonic 270C-II cartridge. I expected levels to match up but it plays pretty loud even with the volume control at the first position (1). The user manual state expected normal listening levels somewhere between 12-17. The other inputs behave like expected. I feel kind of lost right now, anyone experienced something similar with the Quad 34 disc input?
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Check the downstream components. The RIAA and next gain stage are on the main PCB. Someone has been fiddling around.
It's 54k9 in the parts list.
The gain here is set by R21, 100R, on the module PCB, and R22 comes into play.
Maybe you just have a very high output MM?
The gain here is set by R21, 100R, on the module PCB, and R22 comes into play.
Maybe you just have a very high output MM?
The EPC-270C is specified as 3.2 mV output @ vinylengine.com. I made a quick simulation in Tina-Ti of the Quad 34 RIAA stage based on my component values. AC transfer characteristics attached and looks good with some 30 dB gain @ 1 kHz. An input of 3.2 mV should equal 101 mV @ 30 dB from the RIAA stage. First stage on the volume control is -57.20 dB, that equals 0.14 mV with an 101 mV input. I don't know exactly how much more gain is applied downstream but it looks like around 3.3 dB so lets say 0.2 mV on the output. The Quad 405-2 is 100 W into 8 Ohms, that would be some 28 V on the output and equals around 35 dB gain for an input of 0.5 V. A gain of 35 dB applied to 0.2 mV is just 11 mV, or 0.000015 W into 8 Ohms. Did I get the calculations right?
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Regardless of your calculations, the line level in the 34 is 100mV until the volume control, where up to 5x gain is available, making 500mV full output, which is the input sensitivity of the 405 for max volume. So it would be interesting to know the actual output after the first opamp for 3mV 1kHz phono input.
Sorry I meant R21=220R.
Rail voltages have nothing to do with gain other than setting a limit on output.
Does the phono input sound correctly equalized? An RIAA capacitor o/c failure will increase the gain.
Rail voltages have nothing to do with gain other than setting a limit on output.
Does the phono input sound correctly equalized? An RIAA capacitor o/c failure will increase the gain.
Phono input sound correctly equalized yes, just pretty hot (too loud on lowest volume).
My amp (#013298) got no flags installed at R135 and R136. I guess it makes the CD input sensitivity = 300 mV. Its mostly unity gain buffers in the path down to the filters and volume control in my schema. I thought I would start by measuring the CD input to get a feel for the gain in the active volume control. I used a simple DIY tone generator to generate a sine wave @ 1 kHz, 96 mVrms. I then measured the output: 352 mVrms at position 21 (full volume, +11.3 dB), 20.8 mVrsm at position 10 (-13.3 dB) and 2.4 mVrms at position 1 (-32 dB).
I really don't know the best way to measure the phono input (I got no test record etc.). The lowest signal my DIY tone generator can generate @ 1 kHz is 5.6 mVrms. I sent it through the phono input but I don't think its the way to go. Accuracy is probably low (I used my cheap handheld scope) and I am unsure about the readings of the output. It read 8 mVrms at volume control position 1 (+3 dB), 340 mVrms at position 10 (+36 dB) and a full 500 mVrms already at position 12 (+39 dB). Those values doesn't make sense so there must be a better way to do it (or better gear)...
My amp (#013298) got no flags installed at R135 and R136. I guess it makes the CD input sensitivity = 300 mV. Its mostly unity gain buffers in the path down to the filters and volume control in my schema. I thought I would start by measuring the CD input to get a feel for the gain in the active volume control. I used a simple DIY tone generator to generate a sine wave @ 1 kHz, 96 mVrms. I then measured the output: 352 mVrms at position 21 (full volume, +11.3 dB), 20.8 mVrsm at position 10 (-13.3 dB) and 2.4 mVrms at position 1 (-32 dB).
I really don't know the best way to measure the phono input (I got no test record etc.). The lowest signal my DIY tone generator can generate @ 1 kHz is 5.6 mVrms. I sent it through the phono input but I don't think its the way to go. Accuracy is probably low (I used my cheap handheld scope) and I am unsure about the readings of the output. It read 8 mVrms at volume control position 1 (+3 dB), 340 mVrms at position 10 (+36 dB) and a full 500 mVrms already at position 12 (+39 dB). Those values doesn't make sense so there must be a better way to do it (or better gear)...
I did some additional listening tests. I got a DAC and measured it to 288 mVrms with a 1 kHz sine wave. I then (without altering the gain) feed the CD-input with music material from the DAC. The sound level was now more or less the same as from the turntable (when switching between them).
Maybe there is nothing wrong with the phono input sensitivity? It might be that the final amplification by the 405-2 is just too much for my speakers. An input signal of around 100 mVrms at the CD-input is perfect but the turntable is still as hot as with a 300 mVrms at the CD-input.
Maybe the easiest solution is to add a -10 dB attenuation at the inputs of the 405-2 or lower the sensitivity of the 405-2? Anyway, the volume control is pretty useless as it is right now...
Maybe there is nothing wrong with the phono input sensitivity? It might be that the final amplification by the 405-2 is just too much for my speakers. An input signal of around 100 mVrms at the CD-input is perfect but the turntable is still as hot as with a 300 mVrms at the CD-input.
Maybe the easiest solution is to add a -10 dB attenuation at the inputs of the 405-2 or lower the sensitivity of the 405-2? Anyway, the volume control is pretty useless as it is right now...
If the perceived volume of phono and CD are equal at equal settings there is nothing wrong except possibly too much gain. Resolder the 6 connections to the volume control: these are known to go dry.
Check that R6/C4 and R4/C2 in the 405 are as per factory. Somebody may have increased the gain.
Otherwise the easiest solution is to lower the output of the 34. Two resistors per channel. How to do it is described in the user manual.
Check that R6/C4 and R4/C2 in the 405 are as per factory. Somebody may have increased the gain.
Otherwise the easiest solution is to lower the output of the 34. Two resistors per channel. How to do it is described in the user manual.
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R6/C4 and R4/C2 values in the 405 are as per factory. Reading the user manual now... Thanks for your time and support ejp!
Ps. The user manual states "connect additional resistors in parallel with resistors R119 and R122". I got no resistors at the pads labeled R119 and R122. My pads are covered with solder but no 1k resistors to ground as in my schema. So have someone removed a pair of 1k resistors or is this pads reserved for additional resistors to reduce the output? I.e. lets say I want to attenuate output with 15 dB, I should then add 180R in parallel to resistors R119 and R122 according to the manual. The equivalent resistance of 1k || 180 = 150. So should I add 150R or 180R to my empty pads labeled R119 and R122?
Ps. The user manual states "connect additional resistors in parallel with resistors R119 and R122". I got no resistors at the pads labeled R119 and R122. My pads are covered with solder but no 1k resistors to ground as in my schema. So have someone removed a pair of 1k resistors or is this pads reserved for additional resistors to reduce the output? I.e. lets say I want to attenuate output with 15 dB, I should then add 180R in parallel to resistors R119 and R122 according to the manual. The equivalent resistance of 1k || 180 = 150. So should I add 150R or 180R to my empty pads labeled R119 and R122?
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As ejp says just restoring R119 and R122 will reduce the outputs by 15dB , 20 log (( 1K /( 2k7+2K2+1K) )) .
Someone must have been using the 34 with a modern power amplifier and disabled the output attenuator. The 405 has a sensitive input
A quick update, I added 1k resistors to the empty R119 and R122 pads and everything sorted! The volume control is usable again, still quite hot with my speakers but normal listening levels around 8 instead of 1. Thank you ejp for putting me on a path towards R119 and R122. Now enjoying some vinyl on my working pair of Quad 34 + 405-2. 😍
A quick update. It was still a bit hot and I found one additional root cause. Someone had bypassed the balance control with solder. A faulty potentiometer? I de-soldered it and found that the tiny pieces of plastic holding the wipers had broken on both sides. I used super glue to fix them in place and additional epoxy glue for additional strength. Balance control is now working and the output level is finally as expected. My fully repaired 34 + 405-2 is now paired up with a Technics SL-23A and I am really enjoying this setup as my daily driver for vinyl.
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Well done. Those bits of plastic are unbelievably fragile for something that is supposed to last decades. I just hit them with a soldering iron.
For the 405-2, I recommend reducing the gain. That lowers the noise. Good source of information: https://keith-snook.info/quad-405-2002.html