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Resolving cold midrange in 5687 circuit
Resolving cold midrange in 5687 circuit
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Old 14th February 2009, 06:17 AM   #1
phrarod is offline phrarod  United States
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Default Resolving cold midrange in 5687 circuit

I built an kit pre-amp. Its very well designed transformer output but the mids are very cold. I'm not sure where to go to optimize this for a better mid tonality and weight.

A friend suggested changing the .1uF coupling cap with a .22 and dropping the grid resistor from a 1K to a 200 - 400R.

My voltages at the moment are:

Pin 1: 95VDC
Pin 3: 4 VDC
Pin 4/5: 11.75VDC
Pin 6: 13VDC
Pin 8: 5.9VDC
Pin 9: 258VDC

This is a copyrighted production circuit. Hopefully AN kit doesn't mind me showing it.
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Old 14th February 2009, 01:40 PM   #2
Eli Duttman is offline Eli Duttman  United States
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IMO, the absence of a grid stopper on the O/P triode is an error. As has been discussed, at several times and on several "boards", grid stoppers should be Carbon composition.

Before you change the value of any parts, please tell us what sort of coupling cap. is in situ now?
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Old 14th February 2009, 01:54 PM   #3
analog_sa is offline analog_sa  Europe
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What is the step-down ratio? I would be very tempted to remove one of the stages.

What is the PS? Valve rectified? SS rectification may be the reason for the cold midrange.
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Old 14th February 2009, 03:31 PM   #4
phrarod is offline phrarod  United States
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The power supply is a valve rectified ECL82 with a 6x5 output.

The .1 is an Audio Note Tin cap.

On the grid stopper - you mean after the coupling cap? As there is a grid stopper on the input.

Yes, I changed the metal film to carbon on the grid stoppers when I built it.

The OT is a 33:1
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Old 14th February 2009, 03:46 PM   #5
Brit01 is offline Brit01  United Kingdom
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Quote:
On the grid stopper - you mean after the coupling cap? As there is a grid stopper on the input.

The grid stopper should be as close to the tube pin as possible (here between C5 and the pin 7). Preferably soldered directly to pin 7.
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Old 14th February 2009, 03:50 PM   #6
phrarod is offline phrarod  United States
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What about the value of the stopper. I've been doing some searching and seeing more values like 300R instead of the 1K they use. Also all the others use a .22 C

Anyone know where I can find the schematic to Brett's 12B4?

What do you guys think about the voltages on the pins?
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Old 14th February 2009, 03:53 PM   #7
Eli Duttman is offline Eli Duttman  United States
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Quote:
On the grid stopper - you mean after the coupling cap? As there is a grid stopper on the input.
That's correct. The 5687 is a high gm type, which makes it especially vulnerable to parasitic oscillation.

Quote:
Yes, I changed the metal film to carbon on the grid stoppers when I built it.
Carbon film or Carbon composition? Carbon film resistors are spiral trimmed, which makes them inductive. Only Carbon comp. is both non-metallic and non-inductive.

Quote:
The .1 is an Audio Note Tin cap.
From what I've been able to determine, AN Tin foil caps. have a YUCKY polyester dielectric. Think about installing Soviet surplus PIO (paper in oil) parts.

BTW, the 6X5 is a full wave rectifier. That would leave the 6BM8/ECL82 performing regulator duties.
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Old 14th February 2009, 04:02 PM   #8
phrarod is offline phrarod  United States
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Yes, carbon comp. It tried out the new pink ones - Takman - that Partsconnexion sells.

Still curious about the voltages and values of the grid stoppers/caps

I have some ASC .22/400 in the drawer I could sub
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Old 15th February 2009, 12:56 PM   #9
Burnedfingers is offline Burnedfingers  United States
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The M7 schematic can be found here.

http://www.diyparadise.com/simplepreamp.html



Is this kit an actual product of Audionote or some rip off oriental piece of crap?

In the past I built a copy of the Audionote 5687 line stage and was pleased with the result.
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Old 15th February 2009, 01:40 PM   #10
analog_sa is offline analog_sa  Europe
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I have been pondering the 33:1 step down. Why would anyone use such a transformer? Did AN recycle an output?
Seems like all their current kits come with ECL82 as regualtors - dacs and all. While previously even their phono stages used passive filtering. Weird. Must have had an excellent deal on the ECL82s.
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