Spectral DMC 6 Preamp

American Audiophile

New Member
Aug 17, 2010
36
0
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Stuart, FL
With many thanks to Demian Martin to clarify the genesis of the DMC 6 and other preamps in the early Spectral line up. Prior to this there has been much conjecture on these preamps and now we have the truth and iis welcome indeed. Prior to the review I posted I heavily scanned the internet for any info I could find on the DMC 6. So kindly forgive my errors in that review, it was by no means to disparage your fine work and I applaud your coming forth to set the record. Many, many thanks. However I gotta tell ya the DMC 6 is a wonderful preamp and one of the absolute best I have heard in 53 years of this hobby I've been in. Same goes for the DMA 50 amp I have as well.
 

mullard88

Well-Known Member
Jun 5, 2010
948
62
1,588
Hi 1audio,

I'm extremely thrilled to read your inputs here. Hope to read more posts from you.
 

SKATTERBRANE

Well-Known Member
Nov 27, 2011
11
0
78
I have a Spectral DMC6 since new, and an Entec SW-1 and, for my smaller system, an Entec LF-10. (love them!)

I am thinking of replacing my preamp in my smaller system (a PS Audio 4.5) with a Threshold FET 10HL or HE.

Also wondering about a H H Scott 222 integrated amp. Here is my question. Can I use the tape loop on the Scott integrated as a preamp out, and amp in, so I can install the Entec crossover in the tape loop, and listen to any source with the tape monitor engaged processed through the Entec crossover?
 

1audio

Well-Known Member
Sep 17, 2010
31
2
353
SF Bay Area
(Dusting off very old memories) The only problem with connecting the crossover in the tape loop is that there will be no volume control for the bass output since the volume control for the system is after the tape loop.
 

DaveSpectral

Well-Known Member
Aug 30, 2010
28
0
388
Near Chicago
Hi Demian. I am running the DMC-6 with a Sumiko BP Evo-3 cart. It is high output MC. (2.3 MV) I used the KAB gain calculator in the link, to set it accordingly, and I wound up at 42 DB gain. So now, I’m moving to an Audio-Technica OC9-3. It is LOW output at .4 MV. Should I go by the KAB calculator with 58 DB, or should I just turn the Spectral phono gain pots up all of the way. I thought that I read that it has 72 DB of gain total. Any advice you have would be great.

http://www.kabusa.com/pregain.htm
 

1audio

Well-Known Member
Sep 17, 2010
31
2
353
SF Bay Area
I would just run it at full gain. The gain control is in the phono section which means the response will change when the gain changes. The unit has enough dynamic range to handle a high output cartridge at full gain. The volume control is positioned to handle managing the range and keeping the SNR acceptable.
Demian
 

DaveSpectral

Well-Known Member
Aug 30, 2010
28
0
388
Near Chicago
I would just run it at full gain. The gain control is in the phono section which means the response will change when the gain changes. The unit has enough dynamic range to handle a high output cartridge at full gain. The volume control is positioned to handle managing the range and keeping the SNR acceptable.
Demian
Oh, o.k. Thanks again Demian.
Another question: How high up does the phono section go? It doesn't go in to the megahertz also does it?
(LOL I just had to add the word, "phono" in to my spell checker. How times have changed, and not all for the better.):D
 

1audio

Well-Known Member
Sep 17, 2010
31
2
353
SF Bay Area
Thats almost a philosophical question. The Phono section rolls off following the RIAA curve starting at 2120 Hz and continues to about 50-80 KHz and then is flat to several MegaHertz, except the DMC6 may have the hybrid version which works similarly but not the same. The line section is flat to around 1 MHz. What that works out to depends on how you want to define the measurements.

The transducer is the real limitation, not the preamp here.
Demian
 

DaveSpectral

Well-Known Member
Aug 30, 2010
28
0
388
Near Chicago
A few days ago, I popped open my DMC-6. As I knew, it is a Series II. What I didn’t remember, is that it is almost completely different from the pics that I posted here of somebody else’s Series I (inside). Next time I open it, I’m going to take some shots, and get them on here for the record. If it was any more beautiful, it would be Kate Bosworth! It’s like I didn’t know what I had here. I’ll post ‘em soon.
For now, here is Kate. :D LOL http://www.imdb.com/name/nm0098378/
 

codimension.one

Well-Known Member
Nov 21, 2012
3
0
296
How to bias the DMC-10 Delta

Hello Demain, and thank you for designing such wonderful products !

I don't know if you monitor this board, but I have a DMC-10 Delta preamp that I inherited from my father (the original owner), and it has not been used in a few years. I opened it up and all looked fine inside (capacitors looked intact, no signs of any stress internally), so I fired it up and it sounded wonderful driving both a solid state amp and a tube amp. However, I do not want to use the DMC-10 for any extended period without checking the bias and offset values. I have the manual that came with the unit, and the desired offset and bias values are given there, but there is no clue as to where on the circuit board to place the multimeter probes to check these settings. It's clear that there are four sections, and each has two adjustment pots (a flat white pot with a large screw adjustment, and a smaller vertical blue pot with a tiny screw adjustment). However, it's not clear where to measure the bias and offset values on each stage.

Thank you in advance for any help you can offer on adjusting and caring for this superlative piece of equipment !


I thought I should clear up some of the history of the Spectral preamps here. I designed all of the Spectral preamps through the DMC-20. Keith O Johnson took over around that time. The first Spectral preamp was the MS-1, a true kitchen table product. We built 20 or so of those. The DMC-10 was the first full production preamp. The DMC-5 and DMC-6 were value engineered versions of the DMC-10. Perhaps the key differentiator in the designs was the combination of low noise and wide bandwidth. The unusual phono implementation mentioned was actually this: the phono section has about 40 db gain typical for a moving magnet phono section. However it has extremely low noise, typically around 3 nV per root Hz, appropriate for a moving coil input. What I did was add more gain in the high level section, 20-35 dB, to make up for the low levels from the moving coil cartridge. This also gives an enormous dynamic range and very high overload for a phono section. To match the levels of the phono section the high level inputs are all padded down. By keeping the stage gain lower I could maximize the bandwidth of the system.

There aren't a lot of differences in the circuitry between the DMC-10 and the DMC-5 and DMC-6. The DMC-5 has an internal power supply, which brings some hum we never really got rid of. The later DMC-10 versions got a cascoded input, that can be retrofitted to the DMC-5 and the DMC-6. Crosby Audio had a substantial mod for the DMC-10 and the DMA-50 and DMA-100. Only a handful were ever modified and they can't ever return to Spectral, having been substantially changed.

The basic circuitry has evolved and I don't know anything about the current Spectral products. I used a similar circuit with a lot of enhancements the the NuForce P-9.
 

1audio

Well-Known Member
Sep 17, 2010
31
2
353
SF Bay Area
Hello Demain, and thank you for designing such wonderful products !

I don't know if you monitor this board, but I have a DMC-10 Delta preamp that I inherited from my father (the original owner), and it has not been used in a few years. I opened it up and all looked fine inside (capacitors looked intact, no signs of any stress internally), so I fired it up and it sounded wonderful driving both a solid state amp and a tube amp. However, I do not want to use the DMC-10 for any extended period without checking the bias and offset values. I have the manual that came with the unit, and the desired offset and bias values are given there, but there is no clue as to where on the circuit board to place the multimeter probes to check these settings. It's clear that there are four sections, and each has two adjustment pots (a flat white pot with a large screw adjustment, and a smaller vertical blue pot with a tiny screw adjustment). However, it's not clear where to measure the bias and offset values on each stage.

Thank you in advance for any help you can offer on adjusting and caring for this superlative piece of equipment !

Its been years since I looked at one. The basics are as follows:
The bias current is set via a resistor to the positive rail in series with the output FETs. If you are looking at the board with the controls toward you the component layout is very similar to the schematic. The resistors are 100 Ohm resistors that are vertically oriented on the right end of each section. You measure the voltage drop across them. The larger square pot in the center is the bias adjustment. Its easy to goo too far so be vareful. +/-20% makes no real difference inthe setting. The offset trimmer is the small vertical trimmer. its a 20 turn pot. In the center right of each section to the right of the three output fets there are three resistor that sum the currents. At the center on any of the resistor leads where they tie together. is the point where you set the bias.

If you need send me a photo and i'll mark it up with the test points.
 

codimension.one

Well-Known Member
Nov 21, 2012
3
0
296
Demain, thank you so much for your help ! Here is a picture of the board. I see the pots you are talking about. I have attached a "raw" picture of the board for you to mark up, and also a close-up with what I think are the correct resistors marked with an arrow. (Ignore the little red and blue boxes ... that was just an initial guess I had made earlier prior to contacting you.)

To the right of each bank of three output fets I see five resistors. The center one is attached to a visible circuit board trace in the two right-hand sections, but not in the two left-side sections. My guess is that the three center resistors in each bank of five are the summing resistors you are referring to. Would I just measure the offset by checking the voltage between the ground plane (presumably the surface of the circuit board, or the chassis) and the center resistor lead that's facing the fets (or should I work on the far side of those resistors, facing away from the fets ?)

Last question: Am I better-off doing these measurements with shorting plugs in the appropriate inputs as I check each section ? (and presumably the left-hand two sections are the phono stage and the right-hand two are the line stage.)

Thanks so much once again ! If you're ever in Tucson and you want a fantastic Sonoran or Oaxacan mexican feast, shoot me an email and it's my treat !!

Spectral DMC-10 delta board.jpg



Spectral DMC-10 Bias Resistor.jpg

(Ignore the little red and blue boxes ... I think it's the ones with the arrows marked)

Thanks again !
 
Last edited:

1audio

Well-Known Member
Sep 17, 2010
31
2
353
SF Bay Area
Those are the correct points. Good luck!
 

grankin

New Member
Mar 14, 2015
7
0
0
CheeseLand
This review deals with one of the genesis products from Spectral in 1986. This was the second preamp produced, with the DMC 5 being the first. This is a ultra wideband preamp extending to 1.2 MHz. and is one of those empirical designs from the gifted minds at Spectral. This was and remains one of high end audios benchmark products of the past century.

The DMC 6 preamp is powered by a stand alone power supply attached to the main preamp via embilical cord. This was the companion preamp to the DMA 50 and DMA 80 power amp. Finished in a beautiful soft silver front that is quarter inch thick. The all steel chassis is robustly built and has a black textured finish. The rear panel supports gold plated RCA's attached to the chassis itself, nice touch there. The gold plated circuit board is a work of art in itself. very well laid out and using parts of very high quality. Sealed pots for volume and balance control. This is no doubt a serious preamplifier for the discriminating audiophile.

The separate power supply is a hefty item, which should be placed as far from the preamp as the embilical cord will allow. Another nice touch there, to make sure that no transformer interaction can degrade the musical signal.

On the front panel the controls are from left to right;

Input Select: Tuner/CD/Phono/Aux/Tape
Record Select: Tuner/CD/Off/Phono/Aux
Mode: Left/Right/Stereo/Reverse/Mute
Output: which is the Volume Control
Balance: Output to left and right channels.

All these controls are of high quality and have that solid yet silky feel to them in keeping with the status of Spectral and their products.

The all important on board phono section, which is the high light of this preamp is well thought out and a joy to use and hear.

[snip]

There are indeed today very few preamps with on board phono sections and this is the configuartion that appeals to me. I have tried many out board phono sections, but none of them appeal to me, more cables to run and another power source in the chain to deal with. But that is my preference.

Once installed let the Spectral run 24/7 before any critical listening and evaluation took place. In the days to the run up for evaluation it did not dissappoint.

By the above play list a wide genre of music was played in both LP and CD format. Of course being the dyed in the wool analog maven that I am, vinyl was first up for the evaluation.

Could not be impressed more with the Spectral DMC 6 in vinyl playback. This phono section is truly stellar as one would expect from Keith Johnson. Its ability to delver the music with a depth, clarity and outright emotion to be able to get oneself totally immersed in the music. The way it has the ability to seek out the layers of the musical experience and to retrieve those signals and pass them through to the listener is a joy not often encountered. Prior to this I thought the Klyne SK5A preamp with its phono section was the best I had heard. However the Spectral DMC 6 is clearly its equal if not in my opinion surpassing the Klyne by a small margin. Not to disparage the Klyne at all, but the DMC 6 is extraordinarily one of the finest preamps to pass through here in many,many years.

[snip]
When this preamp broke onto the market it was priced at $2,495.00 a hefty sum indeed for 1986. In todays dollars that equates to $4,995.00. Although some 24 years in service, in my opinion still ranks as one of the best audiophile preamps I have had the pleasure to own and hear. One can hurl all the audiophile positive adjectives and still come up short on what this preamp is capable of, even by todays standards.

For some odd reason used prices on the Spectral DMC 6 are quite soft in my opinion, why I have no idea. For instance a Klyne SK5A will command a price of $1,200.00 or more same of the Coda 01p. Threshold FET 10 is in same range of the above. Many of the Levinson and Krell preamps will be in the $1,500.00 range or more. While the used average price of the Spectral DMC 6 is only about $700.00. However the trick here is to find one, they very seldom come up for sale and when they do they are snapped up at damn near light speed - so what does that tell you?

A world class preamp for that kind of money is an outright gift by any standard. Plus Spectral remains in business so parts and service remain available, a valid point in considering any piece of pre owned audio gear. It was often said that the Coda 01p is the Rolex of preamps, that being said then the Spectral DMC 6 ranks as the Cartier of preamps.

If needing a sterling preamp for your system, look no further, prices will not remain this depressed very long on the Spectral DMC 6. This should be on your short lists of preamps and a wake up call.

I have such a thing, have owned it since new, and (unfortunately) I need a preamp with more inputs. The FM tuner, the SACD player, the cassette deck still get used. Would like to pass it along to a new owner, but am not sure what venue might be best to try to sell it in. Any clues gratefully received.
 

jmaestas

New Member
Apr 11, 2015
1
0
0
Spectral DMC 6 Power Supply

Hi all!
I just acquired a DMC 6 without a power supply.. is there anyone one out there with knowledge of one for sale or the schematics to build one? Any info would be greatly appreciated!

Thanks!!
 

Miyabi

New Member
Jan 20, 2016
3
0
0
Moscow, Russia
SPECTRAL DMC-6 manual.

Hello!

Could anyone please let me know, where I can find manual for phono board section.
I have some trouble with right channel on phono input.
Some LF noise about 50-60 Hz is appeared in this channel on any set ups (47 KOm, 100 Om…).

Think, maybe it`s possible to fix the problem having factory settings parameters, step by step to test every unit.
I will really appreciated any support in that case.
Thanks!
 

dmadmc

Well-Known Member
Jan 12, 2017
3
0
106
anyone can share the DMC-5 schematic? I have one that needs repair work. Thank you very much.
 

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