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Old 22nd August 2010, 06:44 AM   #11
davada is offline davada  Canada
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Nicely said Chris.

I have to agree with you 100%. If someone had come into my shop handing me something that looked like this, I would probably hand it back and refuse to service it.

Why someone would want to do this to working amplifier is beyond me. My position is if a piece of equipment is operating so poorly that it needs a mod, then it's probably not worth
doing. Likewise if it is worthy, then it doesn't need it. Excluding factory directed mods of course.

Cheers,

David.
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Old 22nd August 2010, 11:16 AM   #12
pi industriale is offline pi industriale  Italy
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I understand your point of view.
Course is that people who work or have worked for ADCOM.
is normal that you see the changes as a punch in the eye.
My statement, which are points of view would not be offended.
But it is a fact, it is ultimately views.
I do not want to fiercely defend anyone, I merely report what I have been told.
I do not have to prove anything.
The amp was purchased used in cheap, knowing that would be modified and adapted to my needs.
The GFA has a wonderful value for money but has limits is not so perfect as is, this I think you know it.
Think I need a service center tomorrow and is unable to repair it myself?

This is the technical analysis that was done before starting work
Differences between I and II 555.


Are structurally identical in the sense that good and bad that will come on the sound does not change much,
remain the most serious defects, or the Q7 which deals with the big gain in power, with all the limitations that may have a stadium like this,
and the slowness of the transistor amplifier.
These stages, together with the necessary feedback that will, fix the age of technology and topology of the final, and we can not do anything.

That said the differences are mainly three, apparently mostly in favor of the second:

-1) On the output stage II is a triplet rather than a darlington darlington,
resulting in greater stability on difficult loads and increased flow capacity of the current II
I say that unless the II is suitable for connection to bridge (although I still would not recommend connecting to the 555 Bridge as the schema does not consider suitable, and this regardless of ability to generate electricity);
I is the least suitable of the second loads highly complex and / or low.
To Michael in particular, with the current load, I'd say there are no problems;

-2), The II has a feedback to the continuous, stable from a supplement, the I have a filter below to maintain stability at low frequency, failing integrator;
But in the second it has recommended to eliminate the DC coupled;
in the I, the choice is more purist, as the integrator is always a compromise and the AC coupling is theoretically cleaner, however in turn accepts the compromise of a low pass on the back, which I think should be investigated and improved.

-3) II have been implemented on small tuning adjustments, but they seem minimal interventions guided by a capable hand, and then bring some benefit to some (greater regularity and extension of open-loop bandwidth).

At the end of the day, if we avoid overloading the output with difficult loads, even from the standpoint of controEFM, and if we make some small modification, I may consider it equivalent to II.

I do not think a person who does not know what he says or engineer improvised as you want to paint.


At first sight I would say it's worth intervene:
On by-pass which would seem even absent (qualcosuccia's on II)
Capacitor-feedback;
-To determine whether the case is to decouple the masses for the ground loop (as in II);

The input impedance is fixed at 23K 300pf,

We can bring it to 90k without problems as in the second and then lower it again with a potentiometer to adjust the volume,
we are wanting to increase a bit 'gain by acting on feedback to improve the dynamic adaptation to a passive volume control (6db will roll up easily without problems, probably 12db).

So take a passive volume control to try (and then delete the current pre) is very easy, maybe instead of increasing the gain of the final increase output of the CDP, or both.


Obviously I have considered the possibility of changing the EC leveling,
is no doubt that being able to use excellent, such as those linked to 6ohm is significant.

But I think the cost is very high, considering that the amp was purchased at 450 €.

Using CE with low ESR so provides benefits to low frequencies to high,
However, the scheme provides for the final darlington running from a shirt that works at 4mA, the current gain saturation can not be very high, so the ability to provide surge current is limited by the topology used,
then it would undermine much of the advantage of an ability to provide high surge current by the EC.
(Version II uses a triple darlington and in fact exceed this limit).

High frequency but the lowest energy required likely leads to impulse current lower
what unites the fact that the EC worsen with time especially ESL, then it is likely that the old EC its high frequency are scarce,
I have therefore chosen to apply in parallel medium capacity, low ESL, which serves the purpose.

The use of the then EC bypass with small scattered on individual plates, used to reduce the inductance of the wiring, not exactly optimal,
Finally use of MKP, it may be that does not serve (in my opinion are) but because Michael has several, costs nothing, and certainly are not bad.

It is therefore considered the best compromise, applying the spirit that normally guide Michael, whereas in future we will always act on EC, maybe Michael will be optimized when the rest of the system.
Using the bypass with the EC average, just use the EC to 16ohm.



Finally the change of input impedance and gain are suited to the possible use of a potentiometer direct
with an input capacitance of 300pF, a potentiometer 4k7 is already slightly invasive to be considered as a limit value, while the lower limit would place a 1K (depend on the ability to generate current CDP used or DAT)
incidentally also the version II uses an input resistance of 100k.

Change the gain to the feedback of some changes the bandwidth usage,
but rather we speak of feedback driven, and not many 6db,
is also an option, if possible (there) to increase the output level of CDP / DAT.

The transistors used in 555 have a good seal to the second breakdown,
Pass system used previously bootstrap (copied from the famous Ampzilla)
with the release of those BJT was no longer necessary,
are other oversized (4 per class) just to increase your work surface with inductive loads.

Risks can only come if pilot electrostatic highly capacitive.
(The use of voltage regulator would improve the estate, lowering the drive voltage).

The capacitors were then medium-high performance, not high, yet highly reliable and durable, I'd be quite comfortable (85 ° C does not mean nowadays than 85 ° C but cheap capacitors with minimum performance threshold of a sort of entry) .

Pass on the choice of using a darlington and a triple, it would be much larger to make a speech (his is not a choice necessariamenmte a limit, if ever a compromise, imbued on a philosophy, a philosophy which, albeit different, apply today on its end)

The 555 is a low-cost resource for anyone wishing to acquire a powerful and smooth finish without fainting, there are many on the market, so talk about possible upgrades, and recovery can be considered of general interest.
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Old 22nd August 2010, 12:02 PM   #13
pi industriale is offline pi industriale  Italy
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Quote:
Originally Posted by anatech View Post
Hi Michele,
Well, my role is as a regular member who is posting about something I am very familiar with. Whenever a moderator posts in an official mode, we show a policeman's hat (like this ).
-Chris
Hi anatech,
A low impedance controller sounds better than a high-impedance,
But to use it and make the pre perfectly usable in all chains serve two buffer
and is obviously not the case,

Our choice is to use a buffer instead of upstream and downstream of the regulator as usual, but could have an ending fit for purpose
(The input characteristics that must be the same then that should have a finish suitable for a passive preamp)
and we'll use cables suitable
In this way we only have the advantages without the disadvantages.


Adds yet another possible advantage,
the input buffer could allow to skip the stage output amplifier DAC / CDP, which as you know in the economic achievements is a great weakness.


We planned instead to make a single buffer before the pot, because I believe that in our case is an important benefit.

But I do not want to limit any future possibility, in fact Michael at any time with only one hour of work will move the output buffer and remove the resistors parallel to lower the impedance of the potentiometer (relay)
in this case and finds himself exactly one pre standard entirely,
without spending a € (€ 1 or perhaps tin).

Also may, if he wants to experiment by adopting a buffer tube.


The fact that it changed the ending to be suitable for use with a passive preamp then I believe only an advantage,
even here there are a lot of finals like that,
and not only the MKII version of the same finish is done exactly.

We are not here to experiment with cabbage, do exactly what it takes to achieve the aim.
Flavio did the designer professional amplifiers for 20 years, has a precise idea of how an amplifier and what should or should not do.

he solution of the buffer input and low impedance of the potentiometer is true that it is little used,
but just to a name that came to my mind, Bartholomew has used national and considers higher
is not absurd, it has the drawback of requiring a potentiometer on the market there but we can build and then what's the problem. (Aloia at the time used a double in a linear potentiometer, but very uncomfortable suited to maximum effect).

We had a little problem because the unshielded cables pick up noise in low frequency, 50Hz hum, but use of shielded cables out of the preamp is gone.

I consider myself an open minded person and I'm glad your intervention and not even criticism are always ways of improving.

Now some photos of pre:

scansione0001.jpg

scansione0002.jpg

scansione0003.jpg

DSCF1937.jpg

DSCF1947.jpg

DSCF1928.jpg

DSCF2010.JPG

DSCF2016.jpg

DSCF2021.jpg

Many of the changes that are then anatech challenges were made by ADCOM to version MK2

Last edited by pi industriale; 22nd August 2010 at 12:06 PM.
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Old 23rd August 2010, 02:58 AM   #14
anatech is offline anatech  Canada
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Hi Michele,
The work is nice and clean on the volume control, well done. I happen to really like signal relays for switching. The volume control portion is also neatly done and uses relays, similar to the Carver Lightstar Direct (except it uses buffers that can be defeated).

Now for some other comments, I'll do my best to be brief. Keep in mind that the original design of the 555 wasn't perfect. It was pretty good and didn't use "cheap" parts.
Quote:
Many of the changes that are then anatech challenges were made by ADCOM to version MK2
No, that's not true at all. They did make some changes as they normally do once the basic circuit has been designed. I think that Walt Jung designed the MKII voltage amp stage and I'm not really sure which amplifier that Nelson designed. Keep in mind that Adcom tended to have a design done under contract and then modified it further themselves. John Curl even did the basic work on a later really nice preamp (GFP-850?). One thing that the engineers at Adcom understood very well was how to deliver an amplifier that was pretty reliable, sounded much better than it's price point and priced in a range where most people could afford a system. They knew where money could be saved and where they couldn't skimp. The filter capacitors are good industrial capacitors, as are the rest of the parts they used. They understood that there were no real better "audiophile" parts that would perform any better than what they used. Even the capacitors that fail and rot the PCB were purchased from a respected manufacturer in normal production quantities. So you can not accuse them of using inexpensive, poorly performing parts. The flip side of this is that at the time, there were no parts that performed any better for anywhere close to the prices they paid for the parts in use.

Let's be clear about this. I am not defending Adcom out of some type of loyalty or warm fuzzy feelings. I am defending them on a factual basis. They put out a product that didn't pretend to be something it wasn't. It was also priced very fairly in my opinion. Also, doing warranty for a brand creates an intimate knowledge of design errors and places where quality was sacrificed for profit. There are brands I serviced under warranty that I had less respect for after I became familiar with the products, and still others were my respect grew for them. I have serviced almost every brand that exists in North America, not all but close.

Under warranty, we are not allowed "to make the product better" in any way. No bright ideas in other words. They pay people to do the design work, and those people are not the service network. We were lucky in that we were trusted enough to assist with fixes and proposed design work to solve a problem. In the professional markets, we would design and install modifications to address a specific requirement as well. This opened up a channel of communication with some design engineers at the manufacturer. So, we learned an awful lot about audio design and the reasoning behind some decisions made to sell a product. Now that I have not been in that position for many years, I do tend to look at a product a little differently, and I'm more likely to fix a design fault that I know for sure is a design fault. However, it's certainly not open season on older designs. For most hobbyists and service technicians, I completely agree with David (thank you David! ) ...
Quote:
If someone had come into my shop handing me something that looked like this, I would probably hand it back and refuse to service it.
He is exactly right on that score. Otherwise, why would the device be brought into him (or any other shop) in the first place? Food for thought.

Consider this Michele, there are no shortage of people who think they are great designers. No shortage of all. Some even have products they have designed hit the market, but that has surprisingly little bearing on how good they really are. In fact, there are far fewer good designers in audio than you would think. The best designers have a day job where they design using real test equipment in a real design lab with technicians who physically build the prototypes. So that's where you will find most of the engineers who actually do know what they are doing. To add to this, if you take the time to talk with a really good designer, you will normally find a down to earth kinda person who will honestly tell you that some of the things they do are market driven. In other words, the market demands certain components or features. If they are not present, that product will be far more difficult to sell in the market no matter how good it really is. The job of an audio designer seems to be to acknowledge the market trends and include those in his or her design. Through all of this, they do their best to design a product with as few problems as they can after the bean counters have had their way.

In my experience, an aloof "designer", or someone who thinks they are above mere mortals, do not typically create the better designs. It's often the real bad ones that these people create. Also, designers who created classic equipment from the early 30's on up knew precisely what they were doing. Their knowledge on average was more complete than what I saw coming out on the market from the mid 80's and on. If only those earlier designers / engineers had the additional knowledge and types of parts we have today.

New post to continue.

-Chris
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Old 23rd August 2010, 03:52 AM   #15
anatech is offline anatech  Canada
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Hi Michele,
Okay, more response.

Quote:
A low impedance controller sounds better than a high-impedance,
Look, I'm sorry but this is your opinion. It is not a fact and it also depends on the system you are dealing with as to whether this is a good or bad thing. A low output impedance that can deliver the required current levels with low distortion is generally a good thing. However, that low impedance must be maintained all the way to the amplifier (or preamp, recorder or whatever else ...). Once you raise that impedance with anything, the quality is in very real danger of dissipating. A "passive preamp" (volume control in a box) is one of the best ways I know to degrade the sound quality. The only place for any type of volume control, be it a continuous control like a potentiometer or stepped resistors, is either in the control amplifier, or in the amplifier proper. Depending on the type of input circuit(s), you may in fact need to use a buffer circuit of some type. Nakamichi called this buffer "HTA", Marantz did this in 1968 when they designed the model 500 amplifier. I'm sure there are plenty other examples of this.
Quote:
But to use it and make the pre perfectly usable in all chains serve two buffer
and is obviously not the case,
I'm sorry, I don't understand what you mean to say here.
Quote:
Our choice is to use a buffer instead of upstream and downstream of the regulator as usual, but could have an ending fit for purpose
(The input characteristics that must be the same then that should have a finish suitable for a passive preamp)
and we'll use cables suitable
In this way we only have the advantages without the disadvantages.
I apologize, but once again I'm not sure what you are saying.
Quote:
Adds yet another possible advantage,
the input buffer could allow to skip the stage output amplifier DAC / CDP, which as you know in the economic achievements is a great weakness.
Actually, I disagree. By allowing the signal output to be a high impedance, you become open to higher noise levels and other problems. One of the ways that good equipment reduces noise is to have a low output impedance - especially in the preamplifier to amplifier link. Those signals can be at levels approaching what comes out from your MM phono cartridge. It deserves more care than what I normally see in the "high end" brands. Curiously, so called "mid-fi" products tend to get that mostly right.
Quote:
We planned instead to make a single buffer before the pot, because I believe that in our case is an important benefit.
Well, at least you thought about it. I disagree on purely technical grounds, and I also disagree from an experience point of view.
Quote:
But I do not want to limit any future possibility, in fact Michael at any time with only one hour of work will move the output buffer and remove the resistors parallel to lower the impedance of the potentiometer (relay)
in this case and finds himself exactly one pre standard entirely,
without spending a € (€ 1 or perhaps tin).
I wish him luck with this, but you must always acknowledge the rules of physics and electronics. Any time you have a signal "leaving the box" and have to drive a cable, you need to maintain a low impedance, as reasonably low as you can get without adversely affecting other parameters. You always have to look at the entire system when deciding on possible circuit changes.
Quote:
Also may, if he wants to experiment by adopting a buffer tube.
Well, tubes (valves) are generally more noisy than solid state parts are. If he really wants to install a tube in the signal path, it should be installed before the volume control so that at low volumes the output noise from the tube doesn't dominate. One example of doing this wrong can be found in the Counterpoint SA-1000 preamplifier. The tube is run "wide open" and is after the volume control. Guess what? That preamplifier has problems with signal to noise ratio (think - hissssss and microphonics).
Quote:
The fact that it changed the ending to be suitable for use with a passive preamp then I believe only an advantage,
even here there are a lot of finals like that,
and not only the MKII version of the same finish is done exactly.
I am sorry, but your meaning is unclear. But I will say again that the use of any device in the signal path that degrades the overall performance is a bad idea. This is the basic problem as I see it.
Quote:
We are not here to experiment with cabbage, do exactly what it takes to achieve the aim.
I'm afraid that you appear to be using the wrong ingredients for your salad there. For what you are doing, you do need cabbage.
Quote:
Flavio did the designer professional amplifiers for 20 years, has a precise idea of how an amplifier and what should or should not do.
I've commented on this idea earlier, but does my 30 + years in audio electronics count for anything? You are surrounded by members who are full engineers, and others that have a good feel for audio design work. I don't know Flavio, or his work, but from what I see here, I have strong reservations about what his beliefs are where audio is concerned. Sorry. But thats okay, he very obviously disagrees with me just as strongly.
Quote:
he solution of the buffer input and low impedance of the potentiometer is true that it is little used,
but just to a name that came to my mind, Bartholomew has used national and considers higher
is not absurd, it has the drawback of requiring a potentiometer on the market there but we can build and then what's the problem. (Aloia at the time used a double in a linear potentiometer, but very uncomfortable suited to maximum effect).
I missed much of your intent here, but I suppose one solution you are referring to is the use of op amps (from National?). Understand that there are many ways to build a buffer or amplifier. Some involve individual parts.
Quote:
We had a little problem because the unshielded cables pick up noise in low frequency, 50Hz hum, but use of shielded cables out of the preamp is gone.
That isn't surprising to me at all. Lead dress can take a fair amount of time to get right. Having those large capacitors hanging around on the input and feedback areas is not helping you. Keep in mind that wire can receive and retransmit noise from one area to another. Input signals should always be as short and direct as is reasonably possible without throwing it all away by using over-sized parts.
Quote:
I consider myself an open minded person and I'm glad your intervention and not even criticism are always ways of improving.
Well, it's really too bad there is a language problem between us. Some of your ideas don't look too bad as theory, but that board on the voltage amp PCB really needs to go.

I wanted to draw something else to your attention. It concerns your volume control. The input pair of transistors do pass a small current out of their bases to your common ground. Any change in impedance will interact with this base current and create a a signal that might sound like a very small "pop" as you change the volume setting. You may or may not hear this unless you have efficient speakers and a quiet room.

-Chris
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Old 23rd August 2010, 10:07 AM   #16
pi industriale is offline pi industriale  Italy
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Quote:
Originally Posted by anatech View Post
I wanted to draw something else to your attention. It concerns your volume control. The input pair of transistors do pass a small current out of their bases to your common ground. Any change in impedance will interact with this base current and create a a signal that might sound like a very small "pop" as you change the volume setting. You may or may not hear this unless you have efficient speakers and a quiet room.

-Chris

Hi Chris,
You do not like the mods I made on the 555.
For you was better leave it as it was because it was so perfect and I made worse it.
OK, I understand your point of view, I am still a bit skeptical.
I respect your experience and your work for me count.
I think the 555 amplifier is very good amp but after 30 years certain technologies become old and has changed also how to design the amps.
For some things, as the transistor, was not superior technology available such as MOS or BJT today.
It wasn't know the importance of the bypass, after in the 555II were used bypass!

I did not understand what you have in mind for the preamp, what is it?
Anyway, I'm curious.

PS sorry for my english
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Old 23rd August 2010, 10:24 PM   #17
anatech is offline anatech  Canada
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Hi Michele,
Quote:
For you was better leave it as it was because it was so perfect and I made worse it.
Well, no. That is not exactly what I said. And in no way do I think the amplifier is perfect as it is. What I said was that it was very good quality and performance for the price. Can it be improved? Absolutely it can, but I don't think your changes were in the correct direction. There are some things you didn't address at all, and the changes you made were too invasive. The PCB you made for the switches and capacitors must go, it can't remain like that. Certainly, this should not be done to any other amplifiers.

I think that some of the workmanship was very good, but then there are examples of work that should be stripped out and redone properly if you really needed to do those things. Again, that PCB over the original PCB is not well done, but I can see you did go to a great deal of effort to try and do a good job with it. Getting those pins to fit properly wasn't easy. Now, how would you like to remove and replace that board every time you had to work on this unit?

Quote:
I think the 555 amplifier is very good amp but after 30 years certain technologies become old and has changed also how to design the amps.
Well, here again we should be careful about what we are saying. To be perfectly honest with you, audio designers have learned more about what works and what doesn't. But the lessons about what capacitor to use and other similar things had been figured out in the audio world long before this amp was built. In fact, if you go back in history and read about what circuits were being used in instrumentation, you will see most of the basic circuits and all the theory that applies. We are lucky in that newer transistors did improve greatly, but those also existed before this amp did (2SA970 and 2SC1775 for example - and these are still excellent parts!). The only real late improvement has been in the form of "perforated emitter" type power outputs, but then they were using 2SD424 and 2SB554 because they were far better in some characteristics than parts like 2N3773 and 2N5609 and other pairs. In short, nothing much has materially changed in audio design except for a better understanding on how to design for the general technicians and some audio designers. I can show you many modern designs that are decidedly inferior to designs done earlier in time.

For instance, why do we have DC offset servos?
They two main reasons are so that the input diff. pair match isn't as critical for DC offset (but they are for THD, etc) and they have eliminated a procedure on the assembly line, and also in the field. Countless early power amplifiers survived just fine without the servo, but they did need the odd adjustment. Matching the inputs is a manual task that costs a lot in manufacture, but you could see this more when there was an offset adjustment. All they did was to hide that particular evil. Personally, I think that DC servos are normally not done right and are a step backward.

Quote:
For some things, as the transistor, was not superior technology available such as MOS or BJT today.
My personal findings are that mosfets are not as good as BJTs in an output stage. Uncorrected, they have higher distortion and higher impedance. They also do have a breakdown mechanism similar to second breakdown in a BJT, but it's not nearly as bad. It's just that people have the idea that they are indestructible. They aren't.

Quote:
It wasn't know the importance of the bypass, after in the 555II were used bypass!
Yes they did. but, they did it properly without resorting to giant parts hanging out everywhere. That is my point.

Quote:
I did not understand what you have in mind for the preamp, what is it?
I think I was trying to clarify your ideas I could understand it. That and I pointed out that a gain stage that runs after the volume control isn't a good idea, especially not a tube based one. I gave you an example of a preamp that did what you were suggesting.

-Chris
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Old 24th August 2010, 03:57 AM   #18
EchoWars is offline EchoWars  United States
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People send me stuff like this all the time.

The usual request is to 'put it back the way it's supposed to be'.

With this one, I'd be tempted to say 'it's too far gone'.

Edit: Looks like Davada said much the same thing. Sorry...was not just jumping on the bandwagon. It just reminded me of the weird cr@p that gets sent to me all the time. When a piece of audio gear has been around for 30 years, there's much too good a chance that some techclown has gotten his hands in it somewhere along the line.

Last edited by EchoWars; 24th August 2010 at 04:07 AM.
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Old 24th August 2010, 05:17 AM   #19
embrown057 is offline embrown057  United States
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I just finished the restoration of my GFA-555 with a special mention to Anatech for all his help. I was also a technician in Denver back in the 70’s for some time, MER and Solid Sound “The place to go” We did a lot of work for headliners appearing at Red Rocks amphitheater and local musicians. I had seen a lot of mods on Fenders, Marshals, Peavey, and Crown etc., which looked a mess and sounded the same. I am all for modifications if well thought out, effective and clean, stress the clean. I also believe if a tech or anyone that repairs whatever it may be it should look like no one has ever touched the unit. There just seems to be a level of disrespect to the designer and the name placed on the unit to do any other than the best work possible. I would suggest if you really want to improve on an already very good amp stroll over to the trend on the “Spice Simulation of Adcom GFA-555. Mr. “PB2” has spent untold hours researching the improvement of this amp. This is one I have been following for some time and believe me these guys know what they’re doing and have documentation to back it up. They have been doing this for a while and are very well respected in the DIY community
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Old 24th August 2010, 03:16 PM   #20
davada is offline davada  Canada
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Originally Posted by embrown057 View Post
I just finished the restoration of my GFA-555 with a special mention to Anatech for all his help.

Great job.
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