Adcom 555 Upgrade

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Cap:

- bypass with MKP da 1 a 2.2uF (see skem.)

N°12) 47uF 100V 105°C low ESR & ESL

N°4) 470uF 100V 105°C low ESR & ESL

N°2) 4,7uF 100V 105°C low ESR & ESL

N°4) 22NF MKP

- Resistor (see skem.)

Mod:
An externally hosted image should be here but it was not working when we last tested it.



-Replace the 2 resistors for the high input impedance
(As was done in version MkII).
This change only with passive preamp,
does not affect anything with pre activated.

-C3 (47uF 100V) that must be replaced with new ones and bypassed with 2nF.

-increase of 6dB gain,
to do what is needed from another EC 47uF and a 1k resistor
You need to put the EC from 47uF in parallel with the previous C £, and the resistance 1Kohm 1% metal film resistor R5 in parallel to another, always 1K.
An externally hosted image should be here but it was not working when we last tested it.


Thus doubling the value of C3 and halve the value of R5;
ie double the voltage gain of the final (+6 db), maintaining the same low-frequency cut.

(((Note that C3 and R5 are on the contrary, R5 and C3 to the transistor to ground, but obviously is equal))).

-C5, or 4.7uf capacitor in parallel to P1,
should be replaced (but not with the 47uF 4.7uF) and bypassed with 22nF.

-each group (uF MKP 47uF +1) will have a cable that goes to the stellar mass on his own

The most invasive change is the increase in gain of 6dB,
you do not need unless you use the passive preamp.


An externally hosted image should be here but it was not working when we last tested it.


Video:
YouTube - ADCOM GFA555 UPGRADE

Good work
Ciao

Michele
About Adcom 555: Upgrade, possibilità e varie altre considerazioni - Audioreview - DigitalvideoHT - FORUM
 
skem

definitivo.jpg

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I'm not sure I understand your question.

I'm Italian and I do not speak English very well. If the answer were to meet you, please explain better.

To make a passive volume control (and therefore not use a pre) is easy, simply increase the gain of the final.
The input impedance is fixed at 23K 300pf.

We can bring it to 90k as the GFA 555 II without problems and then lower it again with a potentiometer to adjust the volume.
We have increased a bit 'gain by acting on feedback to improve the dynamic adaptation to a passive volume control (6db will roll up easily without problems).

Finally the change of input impedance and gain are suited to the possible use of a potentiometer directly.
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.

Closing switches have +6 dB.
They have the gold and suitable level of signal.

| Relè, switch e indicatori | Interruttori | Interruttori/Selettori a slitta | Segnale

Opening switches back to 0 gain.

CAp
Evox-Rifa | Passivi | Condensatori | Elettrolitici | Assiale 105°C |PEG124PD247VQ

have no other plans than those published.
The schematic of the change of gain is in yellow.

I hope I have answered your question.
Otherwise, I ask you to be more precise for a better response
Suggerisci una traduzione migliore.

Ciao
 
Do you know if these mods would work for an Adcom 585? I have one and know that some have said these can be modded to sound pretty good.

I do not know the Adcom 585 but the bypass of capacitors of power supply i think is possible to make and certainly will improve his performance.
The old electrolytic capacitors lose their original characteristics ESL in particular over time have the same performance as if they were new.
You can add EC 470uF low ESR and ESL + plastic MKP 1uF to high frequencies in parallel to the big CAP of filtration.
I used this system, but also i don't know if it is fits on the 585.
 

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Hi Michele,
I can see you have done a lot of work on this, but I can't recommend anyone follow your example. Using a straight volume control (erroneously called a "passive preamp") isn't recommended for a number of reasons. I know there are some people who would swear by this, but often the real performance can be hampered simply because of the higher impedances the circuit is now working at. Personally, I'll agree that in the odd circumstance, a volume control may not deliver poor performance. But in those situations, you will find that volume control mounted inside the amplifier chassis and wired with normal hookup wire. Shielded wire is not something that will work well at these higher impedances. The source also becomes far more important than it really should be.

Increasing the resistance as you have throws off the DC balance in the input stage. Look at the resistances that each side of the diff pair "sees". They are no longer even close to equal, are they?

The PCB above the input board is blocking the airflow over the pre-driver transistors, so they will run at higher temperatures. Not something you really want. Using larger capacitors raised up also exposes everything to higher noise levels.

Bypassing the main filter capacitors does not achieve much the way it's been done, and Adcom has originally bypassed the supplies where they need to be. You have added a bunch of extra parts that merely acts as an antenna to radiate supply noise further. If your intent is to reduce the diode turn-off spikes, you should be using a damped capacitor. This is also known as a "snubber". An undamped capacitor may actually make ringing worse.

This seems to be a collection of work that is popular amongst hifi enthusiasts who don't really understand how things work. No offense to you because I know you are trying to improve the amp. I just think you've gone in the wrong direction here.

-Chris
 
Hi Michele,
I can see you have done a lot of work on this, but I can't recommend anyone follow your example. Using a straight volume control (erroneously called a "passive preamp") isn't recommended for a number of reasons. I know there are some people who would swear by this, but often the real performance can be hampered simply because of the higher impedances the circuit is now working at. Personally, I'll agree that in the odd circumstance, a volume control may not deliver poor performance. But in those situations, you will find that volume control mounted inside the amplifier chassis and wired with normal hookup wire. Shielded wire is not something that will work well at these higher impedances. The source also becomes far more important than it really should be.

Increasing the resistance as you have throws off the DC balance in the input stage. Look at the resistances that each side of the diff pair "sees". They are no longer even close to equal, are they?

The PCB above the input board is blocking the airflow over the pre-driver transistors, so they will run at higher temperatures. Not something you really want. Using larger capacitors raised up also exposes everything to higher noise levels.

Bypassing the main filter capacitors does not achieve much the way it's been done, and Adcom has originally bypassed the supplies where they need to be. You have added a bunch of extra parts that merely acts as an antenna to radiate supply noise further. If your intent is to reduce the diode turn-off spikes, you should be using a damped capacitor. This is also known as a "snubber". An undamped capacitor may actually make ringing worse.

This seems to be a collection of work that is popular amongst hifi enthusiasts who don't really understand how things work. No offense to you because I know you are trying to improve the amp. I just think you've gone in the wrong direction here.

-Chris

Ok no problem :D Every opinion is welcome too critical.
I respect your opinion but do not share.
I made an external volume control passive, which works through relay controlled by a microprocessor and keeps the impedance 1k fixed. (Very low):) The quality gets the low impedance you should know.
If someone interested I can put photos.
Who has an active pre will not make changes to the gain that is more invasive.

I have not found any kind of noise and that the capacitors can make the antenna is your opinion even if I had never heard before I respect him but I have not found what you say.
For excess heat have not got problems. The ADCOM amplifier and a very powerful and I use a quarter of the power that is capable of delivering so temperatures are always very low and then input tab there aren't special sources of heat.
Could you explain why the condenser bypassing are not good as they are?
I inform you that the change was designed by an electonical engineer who worked for 20 years in the design of professional audio equipment.
So he knew exactly what he was doing.:rolleyes:

I understand your role as moderator, but why judge this without knowing the first things? Say that the change was made without knowing what he was doing is wrong. Or rather it is simply your opinion is not necessarily the reality of things.
 
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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 :cop: ). This is a symbol that moderators use to draw attention to a more official statement. Right now, I am posting as a very experienced technician.

I did authorized warranty service for many brands of equipment which includes the Adcom brand - right to the end and a bit beyond after they were bought out and in transition to the new ownership. They moved the entire company more to the south in a completely different state, which caused all the original employees to find work elsewhere. While doing warranty (and out of warranty) work, we often came across equipment that had been modified in every conceivable way. Very, very few efforts resulted in an increase in reliability or improvement in sound. Of course, that is what the reason for these amplifiers to be in for service in the first place. That and repair attempts that were unsuccessful. I ran that company for 16 years before selling it about 10 years ago. Therefore I have had almost continuous experience with these models and what hackers tend to do to them. I'll try to answer some of your specific questions.

I made an external volume control passive, which works through relay controlled by a microprocessor and keeps the impedance 1k fixed. (Very low) The quality gets the low impedance you should know.
Okay, so how do you know what the actual impedance is of your particular circuit? Are there any active components in there such as buffer(s), or is it simply a switched resistor network through relays? This is an honest question, I am not trying to trip you up in any way.

If your resistive network presents a 1K0 impedance with respect to ground (or common), are you absolutely positive that your signal source can properly drive that low an impedance? The industry accepted standard was 47K (or 50K depending on who you talk to), so a 1K0 load is severe. Even if you can drive that low impedance, do you pay in extra distortion from the source equipment? Many, many output stages claim to be able to drive a 600 ohm load, however they don't always tell you what the performance is at those loads. Often, consumer equipment data sheets are word games.

Now, the last bit in this quote isn't true at all. In fact, low impedance circuits are commonly used in radio frequency cables and circuits. That is simply a method used to get the high frequency bandwidth up there. This is also the same reason why RF amplifiers for consumer antenna or cable run hot, they need the higher bias currents to charge and discharge all the cable capacitance. There are also reasons that have to do with the impedance that transmitter - antenna systems are more efficient at. Note that is is not audio, and that audio signals do not require the same high frequency response that high frequency work does. So just because we need to use low impedances in high frequency work does not mean that this is superior for audio.

If you want to talk about current amplifier technology, that has problems as well. Inductance now becomes the problem that capacitance is for voltage amps. They can work fine, and even be superior in some instances, just as voltage amplification is for other situations. Again, a current amp isn't automatically better for audio.

When you begin messing around with the feedback network, you are also fiddling with the stability of the amplifier (it's true, honestly!). You should have phase angle information considered when changing the feedback level. Also, reducing the level of feedback that little bit often increases distortion, and does little to affect the feeling of "liveliness". Reducing feedback will increase the output impedance of the amplifier (decrease the damping factor). That's about opposite of what people normally look for on the "spec sheet".

I'll begin a new post and continue ...

-Chris
 
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Hi Michele,
If someone interested I can put photos.
Please do, and include a schematic. Only a top and bottom picture showing your construction is really required here.

I have not found any kind of noise and that the capacitors can make the antenna is your opinion even if I had never heard before I respect him but I have not found what you say.
Just because you don't hear anything does not mean there is no problem. You have to investigate with some test instruments, and even try your amplifier in a friend's home near some transmitters.

A capacitor is made by separating two conductive "plates" with an insulating material, and sometimes an electrolyte in conjunction with etched plates. This increases the surface area of the plates and allows for a smaller body capacitor. Do you agree so far? Since there must be an outer plate, and that plate is conductive, the surface area of the outside of any capacitor can equally radiate or pick up any signals that happen to be in the space around your equipment. A high impedance circuit using a capacitor like this is far more susceptible to receiving noise than a lower impedance circuit. If the capacitor in question is being driven from a more low impedance source and sees higher voltages than an input might normally see, it becomes a transmitting antenna. The "intelligence" it transmits may be your music signal, test tones if you're on the bench, or even bursts of RF due to the increased capacitance that is capable of making a circuit unstable. Capacitance to ground can be a really serious problem, or you may even suffer capacitive coupling between output to input. Not good! Worse yet, you can't directly hear this type of problem, although this may cause increased distortion of your music. Something that you can't quite put your finger on, but you sense something isn't right. The rule to follow here is rather simple. Do not use components that do not fit in the space alloted for the original parts, not unless the original is unavailable and a proper substitute is only a little bit larger. Ever notice the ground traces that are commonly run between signal carrying conductors on a PCB? That is to reduce crosstalk, or even feedback. Now consider that those traces they are worried about are on edge, and that edge is very thin. Now, have a look a those axial mount capacitors you have used. Is the surface area a little bit larger, or a great deal larger than PCB trace thickness? This is a problem too large to ignore, and the fact that you were forced to put the extra circuitry on a daughter board, as well as extend the normal leads should drive this point home. Just think, the original part was already working with the signal before the signal even arrives up to your board.

For excess heat have not got problems. The ADCOM amplifier and a very powerful and I use a quarter of the power that is capable of delivering so temperatures are always very low and then input tab there aren't special sources of heat.
Low compared to what? These amplifiers run warm, and if you leave it on all the time the problem is exacerbated. Those four parts on the PCB are attached to a heat sink for a reason - to transfer heat to the surrounding air. If the air can not move easily, the local temperature will rise as you use the amplifier. Capacitors are more sensitive to heat, and a non-perfectly matched pair of input transistors will drift as far as the DC offset and error correcting they do. That is the job of the differential pair on the input. It compares the input signal with a sample of the output signal. If these parts are not well matched, the desired signal is not fully subtracted and the difference that appears between their collectors (in the Adcom case) contains more of the desired signal (common mode), more distorted since the difference signal is not fully extracted. So DC drift is caused by a mismatched transistor pair, and the amount of distortion you end up with also depends on how well the input pair are balanced in the passband of the amplifier.

Could you explain why the condenser bypassing are not good as they are?
Sure I can, no problem.
I already pointed out part of the issues with this. If you are attempting to reduce the high frequency components of possible diode switching, you need to be aware that a high quality capacitor (low losses) may combine with any inductance from component leads or even the inductance of the transformer. This tends to create radiated noise. The way to deal with that is to include some losses in the capacitor to dissipate the energy and kill the "Q" of the coupled components. You may not always need to do this, but if you aren't looking you will never know. Thinking of these components mounted across the main filters as an effective HF filter is questionable. The wires to the voltage amp stage and outputs are too far away. Not only that, but Adcom has already provided on board filtering in every case already. Improving on that will take some additional testing to gage the effectiveness. In fact, the cases once again may possibly act as a radiating surface - depending on whether the grounding used is effective at higher frequencies. At the worst, you now have a bunch of extra components hanging around that are generally in the way when it comes time to service the unit.

You commented that these extra capacitors will restore the performance of older filter capacitors closer to their new state. If the main filters are no longer working as they should (normally not a problem), replacement is the only option. The condition of the main filter capacitors is easy to figure out by simply looking at the waveform across the capacitor as it is running. Running a sine wave tone at the same time into a dummy load will load the supply enough to perform this test.

I inform you that the change was designed by an electonical engineer who worked for 20 years in the design of professional audio equipment.
Whether he is good or not is something I can't (and will not comment on), but you have to understand that in any profession, you have really good people and others that shouldn't be doing what they are doing. I know many audio technicians that are not good at their job, as well as lawyers, engineers and so on. You believe in him, so that's fine. It still doesn't mean this person actually knows what they are doing or not. I say that with all due respect. Has he seen the actual work that has been done?

So he knew exactly what he was doing.
Well, more like he knew exactly what he was intending to do, whether or not these changes are advisable or not is something I have commented on earlier. I'm glad you believe in him, but it appears that he reads the audio press a bit too much.

I understand your role as moderator
I thought I was very clear about this. I am posting as a member who has experience in this area. A great deal of experience complete with the factory guided understanding and service manuals (not any more) that I used while becoming familiar with these amplifiers. I think that many years supporting this product line under warranty, and beyond warranty must count for something. The manufacturer had indicated that they felt I knew what I was doing as I performed warranty service. I had also saved many from the scrap heap due to some very questionable servicing done by other "technicians" (sometimes called "plumbers" in the trade).

but why judge this without knowing the first things?
Been there, seen all of this before and have corrected these "improvements". I've seen more done with these (or any amp really) than what you have showed us. I can't understand why you assume that I don;t know what is going on with these changes. Everything I have posted in this thread comes from direct experience.

Say that the change was made without knowing what he was doing is wrong
If that were the case, why on earth did he even touch this amplifier? I was trained by a few different technicians. One was an Austrian who I hold in high esteem, and what he basically taught me was if you don't know what you are doing, Do not touch the equipment. This is extremely good advice. The other thing I'd add is that no one has the right to experiment on equipment they do not own, and they especially have no business charging for that work. I feel strongly about that.

Or rather it is simply your opinion is not necessarily the reality of things.
Hmmm. That wasn't very nice to say.
You are correct in that my opinion has no weight with you, or your engineer friend. But then again, we already know that simply by looking at the work that has been done.

My only wish for you is that some day you take the time to really learn how amplifiers work, and the roles a real preamplifier plays in a system. Use your common sense and an open mind so you can think for yourself. Only then will you be able to consider topics like these. Your amplifier makes you happy, so that's good. Enjoy it, but refrain from doing this to anyone else. I know, other people are charging for this type of work, but that doesn't change the fact that what has been done here isn't bringing you the performance you think it is. Our minds are great at protecting us when we do very questionable things. I still strongly recommend that no one perform this work on any amplifiers, especially not the daughter boards you made for those larger capacitors.

-Chris
 
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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.
 
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.:rolleyes:


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.
 
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 :cop: ).
-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
 
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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.
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! :) ) ...
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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Hi Michele,
Okay, more response.

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.
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.
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.
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.
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.
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.
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).
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.
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. :)
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.
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.
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.
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
 
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:p
 
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Hi Michele,
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?

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.

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.

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.

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
 
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.
 
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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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