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Old 16th June 2012, 01:29 AM   #41
kevinkr is offline kevinkr  United States
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Default Second Stage Battery Bias

Referring back to the schematic where I showed how to implement battery bias on the lower triode of the choke loaded mu follower I have implemented this in my pair of pre-amps.

First I'll state that measurable differences are pretty close to nil with the exception of a very slight change in operating point.

A couple of points I will mention are is that the RIAA network is now polarized to the tune of ~ -770mVdc which is the negative grid bias.

The dynamic impedance/non-linearity of the 1N4148 has been removed from the signal path in the second stage. The gain is a few tenths of a dB higher.

I had a little trouble finding a spot for the battery holder. The required changes are to replace R11 a 562K with a 1M resistor, and add R12 also a 1M resistor.. Note this is slightly different than in the previous post referencing this mod. See attachment below.

What I can say is that it does seem to make a subtle, but positive difference. Resolution seems very slightly enhanced, the bottom end has gained a little additional definition, and the highs are subtly smoother. Very incremental changes, and perhaps not worth the bother of an additional set of batteries, but I think they will be staying.

Current drain is 0.75uA so essentially battery life should be close to shelf life as in the case of the 12V bias battery on the upper section.

Potentially a worthwhile improvement for the obsessive..
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File Type: png batt_bias_as_built.png (36.8 KB, 551 views)
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Old 16th June 2012, 04:42 AM   #42
Ejam is offline Ejam  Australia
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Default RIAA Network Placement

Kevin

I have seen a few preamps with the RIAA network before the coupling cap/grid resistor as in the Loesch preamp (I know it is split eq) as opposed to yours with it after. Does it make a difference, some think so, any thoughts?
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Old 16th June 2012, 06:52 PM   #43
kevinkr is offline kevinkr  United States
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Hi Ejam,
I prefer it before because it does not form an implicit voltage divider with the series resistance in the EQ network and that allows me to maximize the available gain, also one less component tolerance to be concerned about wrt to equalizer accuracy. It is also just a wee bit easier to calculate the EQ network - not that I didn't long ago write a little basic program to do most of the work for me..

The one definite down side of this approach is that it does require a larger coupling capacitor which drives up cost significantly.

Sonically speaking I doubt it makes one iota of difference, except that the SNR should be marginally better when that resistor is placed before the network due to its minor noise contribution but more importantly due to the attenuation introduced if placed after the network.

Note that with certain capacitor types it is alleged that polarizing them with plate voltage makes for subjective improvements, I'm not sure. I do tend to use fixed bias in the second stage of my designs, but have not definitely been able to answer this question with any design approach. I do tend to use good quality polystyrene capacitors in the equalizer network however.

We're talking very incremental differences and valid arguments can be made for either approach.

I've been doing it this way for about 24yrs as a result of having an early epiphany and realizing that I was throwing away a couple of dB of gain in those early 12AX7A based designs. In fact with high transconductance types using low resistance values and large caps in the EQ a 1M grid resistor shunting the network is not much of a problem, very different from my early 12AX7 based designs where that series resistor was around 274K - 301K. (Can you spell noise?! )

Which Loesch are you referring to? I was privileged to know Arthur in his altogether too short life, and am familiar with Thorsten Loesch's LCR based design as well.
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Old 17th June 2012, 02:17 AM   #44
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Hello Kevin,
Funny you should mention Loesch's LCR design. I'm in need of either separate phono and linestage preamps or an all in one unit. I've been looking at various diy options such as Loesch's LCR as well as your Muscovite design. There's no way I can make a choice other than to trust other people's subjective evaluations such as your description of your Muscovite preamp. So, please indulge me and describe your subjective merits of your preamp with LCR types and others.
Regards,
David
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Old 18th June 2012, 05:40 PM   #45
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by TerribleT View Post
Hello Kevin,
Funny you should mention Loesch's LCR design. I'm in need of either separate phono and linestage preamps or an all in one unit. I've been looking at various diy options such as Loesch's LCR as well as your Muscovite design. There's no way I can make a choice other than to trust other people's subjective evaluations such as your description of your Muscovite preamp. So, please indulge me and describe your subjective merits of your preamp with LCR types and others.
Regards,
David
Hi David,
It's been so long since I had the Loesch LCR phono stage on hand that I couldn't really compare them subjectively and be fair about it. What I can tell you is it adheres closely to the RIAA standard, is quiet, is quite transparent, and has no particular quirks that call attention to it. I designed it to be as neutral and honest as I currently know how, and also to be relatively modest in terms of cost to construct. Compared to my older design (which almost no one here has heard) it is quite a bit quicker, more detailed, and presents a deeper/wider image.

Probably not the sort of information you are looking for, but I'm not too comfortable discussing the subjective merits of my designs as compared to those of others.
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Old 18th June 2012, 07:16 PM   #46
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Quote:
Originally Posted by kevinkr View Post
Hi David,
Compared to my older design (which almost no one here has heard) it is quite a bit quicker, more detailed, and presents a deeper/wider image.

Probably not the sort of information you are looking for, but I'm not too comfortable discussing the subjective merits of my designs as compared to those of others.
Hello,
Thanks for your thoughts. When you say "Compared to my ...it is quite a bit quicker...", the "it" refers to your current design the Muscovite, correct? If so, then I'm in. I also like the fact that it will be cheaper to build. Those LCR units are very expensive.

Actually your subjective ruminations are exactly what I am after since that is all I can ever get over the internet. As long as these thoughts ae born out of experience and are honest, I have no problem in using them to make my diy decisions.
Regards,
David
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Old 18th June 2012, 09:58 PM   #47
kevinkr is offline kevinkr  United States
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Hi David,
Yes I was referring to the Muscovite which I think is a significant improvement over my last design. I'm still learning, but I am very pleased with the both the technical and subjective performance of the Muscovite, and because I was doing this entirely for myself there was no design orthodoxy involved - I literally tried every concept I've wanted to evaluate in this design - it was all for my own amusement.. I've never named a phono stage design before so I guess that says something..
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Old 24th June 2012, 02:01 AM   #48
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Default Muscovite Sound

Having listened to several of Kevin's previous phono stages and now having two listening sessions with the Muscovite, I would suggest that Kevin is being a bit modest here.

I cannot in fairness compare it in detail to other phono stages, but this new design sounds very good and represents a substantial advancement over his older designs, espcially the D3A design, which design I am fairly familiar with. It is very dynamic and clear and extremely quiet while having a high gain. The design is neither terribly difficult or costly to build, so I think it would be a good build to consider for anyone interested in an extremely competent phono stage.
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Old 26th June 2012, 01:53 AM   #49
kevinkr is offline kevinkr  United States
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Evaluated the 6S3P-DR in my pre-amplifier and concluded that I like the 6S3P-EV better, found that the consistency of the EV was better on the mu/transconductance front as compared to the smaller batch of DR on hand.

The DR was somewhat less resolving and more laid back sounding to the surprise of my friend (GTHICM) and me as we listened to both types.. Not what we expected.

I have two close friends who are planning to build their own having heard mine, and a couple of others here who have expressed an interest in doing so. Any questions please feel free to ask here.

I'm really happy with the result..
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Old 10th July 2012, 01:51 AM   #50
kevinkr is offline kevinkr  United States
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I'm expecting the custom Edcor power transformer and standard Edcor 5H choke any day now.

Hoping I will have enough time to build the final supply for this pre-amplifier before I schlepp it to the local summer audio-fest to show it off at the end of this month..

The final design will use 6BD5GT as the pass elements but a 6V6 or 6L6GA/5881 would be suitable as well.

The pre-amp has proven an instrument of rediscovery in that I am rediscovering my vinyl collection and hearing things not heard before - the design has sufficient resolution as well that the differences between the SPU Classic GM E II and SPU Meister Silver GM II are readily apparent..

I'm happy with the overall result, and will report the outcome of the new supply build when done.

The battery bias change in the second stage incidentally is a keeper. (This is the change that requires a single 1.5V AA in addition to the 12V A23 already used in the second stage.)
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