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Old 11th March 2013, 07:18 PM   #181
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For opamp based preamps tone controls are necessary. No matter which opamp is used the sound is always more or less anemic and the only way to correct this is to include (Baxandall) tone controls and to use inverting buffer at the output of these to have signal in phase again.

But if discrete preamp is used (like DOZ preamp) tone controls are probably not needed. Even simpler two stage transistor feedback circuit is enough (without active CCS like in DOZ preamp).

But the real killer circuit would be to build Elliott P12 (Figure 2 circuit!) with just selector switch and pot at the input. This is a kind of current feedback version of P3A and if gain is set somewhat higher (35-40dB) this would be the simplest big league integrated ever. Pity that Rod does not offer P12 (figure 2) boards.
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Old 23rd March 2013, 05:39 AM   #182
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Quote:
Originally Posted by ivanlukic View Post
I also prefer low level listening so this is a dilemma for me: either to make gain in DOZ lower (6dB perhaps) or to put a pot at the input...
I am surprised any preamp gain is actually necessary for everyday low-to-moderate volume (?)

I have been listening to my freshly-built P3A on and off for a couple weeks, and with a 2V source, the signal has to be attenuated quite a bit at the source just to make it listenable. Even with some sub-optimal parts, and small power supply, there seems to be plenty of power and a decent dynamic range, into 8-ohm (nominal) speakers.

I am using the 35V rail version, which is rated at 60-65W into 8 ohms, and about 1V sensitivity.
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Old 23rd March 2013, 06:14 AM   #183
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PMI,

The need for preamp is not subjective loudness level related. You are right that if there is enough gain in P3A, line level source (like CD) will give sufficient signal. So, 27dB gain in P3A is OK for passive preamp. But most people are not satisfied with the sound of passive preamp, sound is usually dull, too laid back, no detail, somehow dynamic is lacking. Even opamp based preamp will not remedy this unless with tone controls. The only solution, for my taste, is to build minimalist discrete current feedback preamp, like DOZ preamp, with small gain 6-10dB.

As I said, if one decide to build P12 circuit, the sound of DOZ preamp will be present in the power amp and no preamp will be needed. The sound will be very much like P3A with DOZ preamp. Higher gain than 27dB will be needed only to be sure that even insufficiently low input signal will be able to produce max power from the amp.
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Old 23rd March 2013, 08:21 AM   #184
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This all is the question of taste. If you like max resolution, detail, speed, great soundstage, emotion, involvment, excitement, than voltage feedback circuits are not enough. VF circuits, like P3A, have some practical qualities: distortion is low, there is no complicated setting of DC offset, power supply rejection is excellent. Specs are OK with VF circuits. But the sound is less satisfying. They need something to give them excitement. It is current feedback preamp they need. It is not solution to build discrete preamp with LTP input, because you have just another opamp. It must be current feedback to give that open character to sound. I would always chose circuit that has higher distortion but gives better subjective impression. CF circuits have little more distortion (second harmonic) than VF circuits but are much better in bandwidth, speed, phase response. This makes them better sounding.

Therefore when you pair VF power amp (P3A) with it's practical qualities and CF preamp (DOZ) with its good subjective character, you get the best of the two worlds! Or more simple solution: CF power amp (P12) with passive preamp.
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Old 23rd March 2013, 09:08 AM   #185
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Quote:
Originally Posted by ivanlukic View Post
Therefore when you pair VF power amp (P3A) with it's practical qualities and CF preamp (DOZ) with its good subjective character, you get the best of the two worlds! Or more simple solution: CF power amp (P12) with passive preamp.
Hi Ivanlukic, what is a P12 CF amp, I cant find anything about this amplifier, A link would be good.
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Old 23rd March 2013, 10:17 AM   #186
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P12 is power amp described on the Elliott Sound Products (ESP) site, Projects pages, Project 12, "Simple current feedback power amplifier". The version I like is Figure 2 version (without output cap).

Simple 60 Watt Power Amplifier
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Old 23rd March 2013, 10:30 AM   #187
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Thanks, Im not familiar with it, there is very little on the web or this site regarding this amp. You obviously rate it highly.
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Old 23rd March 2013, 10:54 AM   #188
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Fifty years ago there was much more solid state CF (=current feedback) amps similar to P12. This is another variation on the well known theme. One example is John Linsley Hood amp described in HiFi News and Record Review in 1980.

The reasons why I like this particular Rod Elliott's P12 (Figure 2) circuit is:

1. Bootstraped VAS stage = more robust, more reliable
2. Complementary feedback pairs (Sziklay) in output stage = more thermaly stable and more linear than full Complementary Darlington output

The only minor issue with this kind of amp is that you need to turn the amp on for a couple of hours before final adjustment of DC offset and bias current is performed. But since we are diy-ers, and not factory that produces thousands of amps, this is really minor isssue.

For this kind of circuit you do not need very fast driver and output transistors, unless you really want your amp to have excellent specs. It is topology itself that gives it excellent sound.
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Old 23rd March 2013, 07:31 PM   #189
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@ivanlukic:
You are correct, I misunderstood what you meant. I think from my little experience, you would not be disappointed by the dynamic range and the clarity of the P3A. Mine is far from working completely to my satisfaction, but the quality of the sound is undeniable. I am not so sure about the ps rejection, my first build seems to have some kind of issue there.

I do not see an obvious reason why a volume pot at the input of a P3A would change the sound for the worse, but I think I may try it once the power supply issue is sorted.

However, it probably will not sound the same as the P12. Better or worse, is very subjective, or so I found in the past.

The P12 seems not so hard to build as a prototype, with or without a PCB. However, I noticed that the circuit has not been built by Rod, even as a prototype, and there are some warnings about component values yet to be determined.

Since the P12 and P3A have a lot in common (similar components), the only way to know for sure, is to build both, listen and compare.

Or, even better, build both, and send them to Sakis to compare...
(That whole compare thing just gives me headaches, anyway!)
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Old 24th March 2013, 07:57 AM   #190
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I am 50 years old and a bit lazy to learn PCB design programs just to design a single pcb but some forum member who already uses such program can design P12 board. I think that component values from the schematic will do the job even without any compensation cap, but the place on the board should be reserved just in case that some comp is needed.

I must say that CF amps subjectively do not have such massive bass, and if you listen only techno, dance, electronica, etc., it is probably better to stick to VF amps.
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