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Old 17th August 2010, 01:52 PM   #1
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Default Adcom 555 Upgrade

Cap:

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

N12) 47uF 100V 105C low ESR & ESL

N4) 470uF 100V 105C low ESR & ESL

N2) 4,7uF 100V 105C low ESR & ESL

N4) 22NF MKP

- Resistor (see skem.)

Mod:
Click the image to open in full size.


-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.
Click the image to open in full size.

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.


Click the image to open in full size.

Video:
YouTube - ADCOM GFA555 UPGRADE

Good work
Ciao

Michele
About Adcom 555: Upgrade, possibilit e varie altre considerazioni - Audioreview - DigitalvideoHT - FORUM
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Old 17th August 2010, 02:52 PM   #2
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Default skem

definitivo.jpg

3.jpg

2.jpg

4.jpg

DSCF1894.jpg
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Old 21st August 2010, 04:54 AM   #3
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I am curious, what beside the added gain did this modification accomplish. I do not see any documentation or published specs for the changes you have made.
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Old 21st August 2010, 06:57 AM   #4
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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.
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Old 21st August 2010, 10:48 AM   #5
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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.

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

Opening switches back to 0 gain.

CAp
Evox-Rifa | Passivi | Condensatori | Elettrolitici | Assiale 105C |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
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Old 21st August 2010, 04:00 PM   #6
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Quote:
Originally Posted by chefjeffrey50 View Post
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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Old 21st August 2010, 10:18 PM   #7
anatech is offline anatech  Canada
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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
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Old 22nd August 2010, 12:33 AM   #8
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Quote:
Originally Posted by anatech View Post
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 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.

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.

Last edited by pi industriale; 22nd August 2010 at 12:46 AM.
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Old 22nd August 2010, 02:04 AM   #9
anatech is offline anatech  Canada
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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 ). 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.

Quote:
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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Old 22nd August 2010, 04:19 AM   #10
anatech is offline anatech  Canada
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Hi Michele,
Quote:
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.

Quote:
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.

Quote:
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.

Quote:
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.

Quote:
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?

Quote:
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.

Quote:
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).

Quote:
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.

Quote:
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.

Quote:
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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Last edited by anatech; 22nd August 2010 at 04:20 AM. Reason: spelling
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