Mf A 370

[ingemar]

Hi there all!!
There are lm 318 op amps in my a370,as i understand they are not the best choice,Mr jacco was kind enough to send me a lot of information about the 370 including a opamp that is interchangeable with no modification,ive had a pc breakdown and lost all the info from mr Jacco.Sombody who knows the name of the opamp??
Best regards Ingemar

[parsecaudio]

Hi ingemar
When a critical substitution like this are made,you must control with a scope if the circuit oscillate and if this happen a compensation cap must be added.
I have made some similar substitution on italian '80 power amp(REVAC) whith a similar circuits(op amp in the first stage) and I have had to modify the compensation.
The changes in the amp:

amp 1 LM741>NE5534
amp 2 LM301> AD711
amp 3 LM308>TLE2141
amp 4 LM308>LT1028

The more critical substitution has been the last one with LM308 substituted with LT1028

[ingemar]

Hi and thanks for your reply.
Ive been reading a lot about this bb 627,was a guy here on this forum who changed from this 318 to bb627 but it was on a MF 170,but if i understand all he did was to cut out legs 1,5,8 thats it.But ok thats maybe to be in the riskzone.
Regards Ingemar

[jacco vermeulen]

Ingemar,

frankly, i can not remember which type i mentioned.

The LM318 has general purpose opamp pin connections. Anything from TL071, NE5534, OPA134 to AD797.
A big improvement would be to step up to a fet input opamp, LM318 isn't.

I believe some have swapped LT1028 for the LM318 in both MF power as pre amps. I've used that one for MC/RIAA stages, active filters and CD player mods. Per Anders can tell you the LT1028 is best at high gains, so keep it for phono stages.
The OPA627 and OPA637 are my favorite opamps since i started using them in 1992.

The LM318 is internally set for unity gain, therefore the MF circuitry doesn't have any components on the board for setting the opamp gain. If you are using other opamps you have to add something to set the gain, at least a resistor between pin 6(output) and pin 2(- input).

If you wish to keep the circuit as is, gain of the new opamp should be set to 1.
It is possible to set the gain higher, at 5 or higher you could use the superfast Burr Brown OPA637. But that would require other modifications to the overall feedback gain circuitry of the A370.
Not something i would recommend.

[parsecaudio]

Hi jacco
The Linear LT1028 and 1128 are unity gain stable,
1028 for inverting of -1 and 1128 for +1...

[jacco vermeulen]

I'll be the last to advise Ingemar to use an LT1028 in place of the LM318. (the LM318 slewrate number is +50V/uS)
I'm interested which substitute P-A would mention.

(internal unity gain setting is not the same as unity gain stable, i think. OPA627 is also unity gain stable but the gain needs to be set externally)

[parsecaudio]

Ok jacco
but every time that an important change like this are made,a stability control must to be made.
The stability of Musical Fidelity A370 are not one of his best features(very high NFB)...

I have used the LT1028 for my audio sine wave generator. I can confirm to you that this I.C is unity gain stable.

The LM318 is a high speed precision op amp..

[jacco vermeulen]

Ingemar can download oscilloscope software and use the soundcard of his PC for a simple check if the amplifier is oscillating.

I agree that a proper scope is a good investment for fooling around, even for an opamp switch. Certainly at the current give-away price level of second hand oscilloscopes.
In my tight budget student days i was dreaming of saving 1000 bucks for a decent used Philips scope.
For measurements i had to depend on university lab equipment access or the help of my pro-electronics audio buddy and his gear.
Nowadays i'm beginning to resemble a Tek collector.

[parsecaudio]

A good idea,Jacco.
Take care to input level in order to avoid the damage in soundcard for too high level.An attenuation circuit is recommended.

[ingemar]

Thank you guys to spend time to answer my questions!!!!

Regards Ingemar