Robets Cadet III PU Adapter
Hi
I've a Roberts Cadet III preamplifier with a module plugged onto the side of it called a PU Adapter. I think its some kind of impedance matching thingie for a particular type of cartridge. Its silver, and inside there is (for each channel) an electrloytic capacitor, colour coded GREEN BLACK VIOLET YELLOW - 50pF? wired in parallel with a 330k resitor. The phono input appears to connect to one end of this and the output goes via a 1M resistor to - I think - the grid of an ECC807 valve, having passed through the input selector switch.
Does this make sense? I'm a total newbie to all this (although I do have some electronic experience - obviously not enough 'cause I can't read the value of the capacitor even after researching it on the net..). The amp is working at line levels and a signal can be heard through the phono stage but it is very weak.
I'm wondering if it's the 1M resistors that are causing this. I might have the adapter for a crystal cartride. Is it worth switching these resistors for - say - a 500K resistor and seeing if it improves things? I don't want to damage the valves as I understand ECC807s are hard to get hold of these days.
Hi
I've a Roberts Cadet III preamplifier with a module plugged onto the side of it called a PU Adapter. I think its some kind of impedance matching thingie for a particular type of cartridge. Its silver, and inside there is (for each channel) an electrloytic capacitor, colour coded GREEN BLACK VIOLET YELLOW - 50pF? wired in parallel with a 330k resitor. The phono input appears to connect to one end of this and the output goes via a 1M resistor to - I think - the grid of an ECC807 valve, having passed through the input selector switch.
Does this make sense? I'm a total newbie to all this (although I do have some electronic experience - obviously not enough 'cause I can't read the value of the capacitor even after researching it on the net..). The amp is working at line levels and a signal can be heard through the phono stage but it is very weak.
I'm wondering if it's the 1M resistors that are causing this. I might have the adapter for a crystal cartride. Is it worth switching these resistors for - say - a 500K resistor and seeing if it improves things? I don't want to damage the valves as I understand ECC807s are hard to get hold of these days.
Hi,
That adapter is probably for a crystal or magnetic cartridge.
As I don't have a diagram for it I can't tell for sure.
I believe however the Rogers Cadet is similar to the Brimar VP10.
Those ECC807s are hard to come by indeed...
Maybe our fellow member 316A can sort this out for you as I believe he knows quite a bit about the Rogers range.
Sorry I can't be of more help to you,
That adapter is probably for a crystal or magnetic cartridge.
As I don't have a diagram for it I can't tell for sure.
I believe however the Rogers Cadet is similar to the Brimar VP10.
Those ECC807s are hard to come by indeed...
Maybe our fellow member 316A can sort this out for you as I believe he knows quite a bit about the Rogers range.
Sorry I can't be of more help to you,
Well whaddya know....?
I've shorted out the 2x1M resistors and now have a very good gain indeed. Bit too good, really; the volume goes about halfway before distorting, so I guess 500k is the value I need (or 470k I suppose).
However, its a bit bass light compared to the line input and I wonder if I've accidentally spoilt the RIAA bias circuit's workings.
On the other hand, I am using a Marantz CD 63 Ki-Sig for a comparison and that player is known for a good bass reproduction. The only way to compare and know for sure is probably to get a line out from my tranny amp and play the turntable through that.
If anyone can advise I'd be glad to hear from you.
Thanks
Jon Bradbury
(PS : Next up : a slightly burned out Audio Innovations series 500)
I've shorted out the 2x1M resistors and now have a very good gain indeed. Bit too good, really; the volume goes about halfway before distorting, so I guess 500k is the value I need (or 470k I suppose).
However, its a bit bass light compared to the line input and I wonder if I've accidentally spoilt the RIAA bias circuit's workings.
On the other hand, I am using a Marantz CD 63 Ki-Sig for a comparison and that player is known for a good bass reproduction. The only way to compare and know for sure is probably to get a line out from my tranny amp and play the turntable through that.
If anyone can advise I'd be glad to hear from you.
Thanks
Jon Bradbury
(PS : Next up : a slightly burned out Audio Innovations series 500)
I had a Cadet IIII as as a student and I'm sure the MM adaptor was Gold.
I also happen to have a Cadet III pre-amp in my cupboard.
Gold coloured disc again.
Its its wired symmetrically , pins 1 to 4 away from the earth (0).
resistor : blue - grey- orange - gold : pins 1 and 3. (68K 5%)
resistor : voilet - red - yellow - gold : pins 2 and 3. (720K 5%)
capacitor : polystyrene - 680P - 5% - 350V : pins 2 and 3.
capacitor : polystyrene - 2500 - 5 - 400V : pins 2 and 4.
I assume the 1st capacitor is 680pF.
I think (and the size agrees) for the second capacitor the last
digit is a power of 10 multiplier for PF = x1 = 250pF.
The 5 I assume is 5%.
sreten.
I also happen to have a Cadet III pre-amp in my cupboard.
Gold coloured disc again.
Its its wired symmetrically , pins 1 to 4 away from the earth (0).
resistor : blue - grey- orange - gold : pins 1 and 3. (68K 5%)
resistor : voilet - red - yellow - gold : pins 2 and 3. (720K 5%)
capacitor : polystyrene - 680P - 5% - 350V : pins 2 and 3.
capacitor : polystyrene - 2500 - 5 - 400V : pins 2 and 4.
I assume the 1st capacitor is 680pF.
I think (and the size agrees) for the second capacitor the last
digit is a power of 10 multiplier for PF = x1 = 250pF.
The 5 I assume is 5%.
sreten.
Page 2 of this thread has the preamp schematic and shows the PU adapter socket (power amp schematic is on page 1).
http://www.diyaudio.com/forums/showthread.php?s=&threadid=45796&perpage=10&pagenumber=1
Note: with the selector switch in the DISC position then:
pin 4 of the adapter is hooked to the input socket (hot)
pin 2 of the adapter is hooked to the grid of the input triode (for conection of feedback and input)
pin 1 is connected to the cathode of the 2nd triode via 40uF (C3). this is a low impedance drive of the feedback signal.
pin 3 has no external connection
The GOLD adapter is the Magnetic Cartridge Adapter.
The SILVER adapter is the Ceramic Cartridge Adapter
For the GOLD adapter (from posts above) feedback network is 68K to 720K in parallel with a cap which you thought was 680pF.
These should be doing the 50Hz (3180us) and 500Hz (318us) timeconstants for the RIAA response. If that were in fact the case then the cap should be about 4700pF??? Hummm -I'm lost too.
Hey SuppersReady - any chance of posting the adapter schemes here??
My Cadet III has the GOLD adapter ONLY.
Cheers,
Ian
http://www.diyaudio.com/forums/showthread.php?s=&threadid=45796&perpage=10&pagenumber=1
Note: with the selector switch in the DISC position then:
pin 4 of the adapter is hooked to the input socket (hot)
pin 2 of the adapter is hooked to the grid of the input triode (for conection of feedback and input)
pin 1 is connected to the cathode of the 2nd triode via 40uF (C3). this is a low impedance drive of the feedback signal.
pin 3 has no external connection
The GOLD adapter is the Magnetic Cartridge Adapter.
The SILVER adapter is the Ceramic Cartridge Adapter
For the GOLD adapter (from posts above) feedback network is 68K to 720K in parallel with a cap which you thought was 680pF.
These should be doing the 50Hz (3180us) and 500Hz (318us) timeconstants for the RIAA response. If that were in fact the case then the cap should be about 4700pF??? Hummm -I'm lost too.
Hey SuppersReady - any chance of posting the adapter schemes here??
My Cadet III has the GOLD adapter ONLY.
Cheers,
Ian
sreten said:I had a Cadet IIII as as a student and I'm sure the MM adaptor was Gold.
I also happen to have a Cadet III pre-amp in my cupboard.
Gold coloured disc again.
Its its wired symmetrically , pins 1 to 4 away from the earth (0).
resistor : blue - grey- orange - gold : pins 1 and 3. (68K 5%)
resistor : voilet - red - yellow - gold : pins 2 and 3. (720K 5%)
capacitor : polystyrene - 680P - 5% - 350V : pins 2 and 3.
capacitor : polystyrene - 2500 - 5 - 400V : pins 2 and 4.
I assume the 1st capacitor is 680pF.
I think (and the size agrees) for the second capacitor the last
digit is a power of 10 multiplier for PF = x1 = 250pF.
The 5 I assume is 5%.
Hi,
somerthing wrong with the above ?
The above is one channel the other is mirror symmetrical.
/sreten.Hi Sreten,
Mine is gold also - I measured rather than took note of markings - I get the same as you except instead of 720k I get 120k between pins 2 & 3.
I must open it up again & look at res colour bands. My res could be badly out of spec or your eyes could be - are you shure the first band is violet & not brown?
John
Mine is gold also - I measured rather than took note of markings - I get the same as you except instead of 720k I get 120k between pins 2 & 3.
I must open it up again & look at res colour bands. My res could be badly out of spec or your eyes could be - are you shure the first band is violet & not brown?
John
Thank you all for the information. I had got the component values from another forum, but found that the sound quality wasn't very good. They said that the cap between pins 2 and 4 was 2500 rather than 250pf. I will make another with the nearest available, 220, and let you know how it works.
Thanks again
Paul
Thanks again
Paul
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