|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
73.147.174.227
I am posting over here rather than in my usual haunting site (Planar) or in Amps as you guys seem to have the much more technical knowledge. Thanks to all for the help on the ARC SP-16.
I have been trying to redraw the schematic for my Music Reference RM-5 preamp that I have owned since 1981. More as a learning event -I will eventually run a Spice simualtion just for the hell of it. It uses 3 6DJ8s - two for the phono stage and one for the line stage and is a pretty simple circuit.
So while tracing things, I find that the right phono input does not have the 47K5 load resistor grounded - it goes nowhere! I have no idea if it never was grounded or if it had a jumper wire (I suspect that's the case) that I accidently removed when I .installed new phono jacks a few years ago. In any case, what amazes me is that I did never noticed it sonically other than I thought there was a 1-2 dB decrease in one channel (can't remember if it was L or R) which I just compensated with the balance control. It was one of the reasons I got the ARC SP-16
I grounded the resistor and eventually will plug the pre back into the system.
So to the guru's: What can be expected by not loading the phono input?Have I been deluding myself that the system sounded pretty good and I wouldn't know if I was listening to a high end system or a clock radio?
Follow Ups:
If your phono input did not have its 47k-ohm load connected, it probably had much higher impedance -- perhaps as high as 1M-ohm. Since you were using a step-up transformer, this would cause a high-frequency peak that would probably not be audible, so you might not hear any difference. It would also cause that channel to play a little louder.
Below is a frequency response graph of a CineMag 3440A SUT terminated with 1M-ohm in the upper trace producing a noticeable peak at 38-kHz and terminated with 47k-ohms in the lower trace. The signal with 47k-ohm termination is about 1.5-dB lower in volume.
Thanks John. At least I haven't been deluding myself for the last 6 decades. Although my upper limit is about 13-14 KHz (not bad for 70) and missing the effect is excusable.One other the question -- the schematic I have (which has clear errors) shows a bypass cap across the resistor (150 or 180 pf) but these were never Installed on the PCB. I know the designer (Rodger Modjeski) used a Denon 103 so maybe he found the cartridge sounded better without the cap. What is the effect of including capacitance loading on the cartridge? And yes I use a SUT (26db) for a DV 17D2 without any parallel resistance across the primaries.
Edits: 10/23/15
I would recommend leaving out the capacitors, especially when using a step-up transformer. The SUT reflects capacitance back to the cartridge multiplied by the square of its turns-ratio. Therefore, 180-pF on the transformer secondary will be reflected back to the cartridge as 180 x 26 2 = 0.122-uF. This is why you should always use a short, low-capacitance interconnect to attach the SUT to your phono stage. Capacitance on the secondary of a step-up transformer is not at all desirable.
Best regards,
John Elison
Great, one more thing not to worry about. l
The cap is for MM cartridges, which often specify an optimum load capacitance (cable plus auxiliary cap plus the tube's Miller capacitance - about 50pF for a 6DJ8). With an SUT, lower capacitance is better unless the SUT maker has other ideas.
In both cases there is a large contingent who feel that any added capacitance causes a sonic loss. I'm agnostic at this point.
This is the grid-to-ground resistor on the input stage signal grid? AFAIK, this resistor helps set the idle bias of that half of the triode (if you are using a duo-triode, in the first stage).
No this resistor goes from "hot" to ground to load the cartridge. There is a 47 ohm resistor with a ferrite ring around one of the leads to cut down EMF that goes from the phono input in series with a 10 ohm grid resistor. I believe I removed the ground connection inadvertently when I replaced the on- board input jacks.This the first stage but there is no grid-ground resistor for self-biasing I assume there is a fixed bias but I haven't gotten that far along to figure it out. The other two tubes have 1 Mohm resistors.There is a 1K resistor running from the B+ to the cathode circuit (and a 22K plate resistor)for all three tubes. Trying to wrap my head around that as well.
Edits: 10/22/15 10/22/15 10/23/15 10/24/15
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: