Op-amp Tweaking

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mojo
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Op-amp Tweaking

Postby mojo » Wed Jul 11, 2018 10:34 pm

Yes, I know - it's supposed to be called "rolling", and the "rolling your own" analogy sort of works with cylindrical things like valves, but it doesn't really with op-amps, IMO.

Anyway, I recently bought a pair of Hypex NC500-based monobocks by Apollon. Originally I wanted to make my own amps using NC400 modules, but (crazily) it was actually cheaper to buy ex-demo ready-built units from Apollon - with Sparkos op-amps included in the price - than to buy the parts without fancy op-amps, direct from Hypex as a kit. Even buying the parts separately and using the Ghent Audio cases would have made minimal savings!

Overall I'm very pleased with the Apollon amps, but the one thing I didn't expect was to find that the bass with ATC SCM40s has become sightly overbearing, compared with the Smartpower+PSX-R that I was using before. I had expected the nature of the bass to change a little, but not the balance between bass and higher frequencies.

My initial reaction was that perhaps the cause is the Sparkos op-amps. They are described as "warm", but that does not seem to translate to any significant variation in gain across the published frequency response - so why would that lead to bass emphasis?

At short notice I can't get hold of the main alternative (Sonic Imagery 994's) but what I did try was to put in a pair of LM4562 op amps instead of the Sparkos ones - these being £2 each in small quantities (probably more like about 40p each in industrial quantities), and the op-amps that Hypex themselves use in their own buffer boards. And... try as I may, I can't hear any meaningful difference between the two!

So my question - has anyone else here done any such op-amp tweaking and, if so, was your tweaking any more rewarding than mine?

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Re: Op-amp Tweaking

Postby angelface » Thu Jul 12, 2018 8:07 am

mojo wrote:They are described as "warm", but that does not seem to translate to any significant variation in gain across the published frequency response - so why would that lead to bass emphasis?


I think warm often means more bass. I certainly got more when I went SP+ to Mono X. I do think bass needs power (perhaps more current than watts) so going for a more powerfull amp might appear to give you more bass.
Last edited by angelface on Thu Jul 12, 2018 10:35 am, edited 1 time in total.
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mojo
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Re: Op-amp Tweaking

Postby mojo » Thu Jul 12, 2018 8:55 am

That's the problem with subjective terms like "warm" - in sonic terms it doesn't mean the same thing to any two people!

Some reviewers have mentioned slightly different bass with the Sparkos op-amps, but mainly they say the difference is very subtle, so perhaps it's too subtle for my ears...

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Re: Op-amp Tweaking

Postby Ianmac » Sat Jul 14, 2018 6:36 am

Can you be sure its an op amp issue ?
Ive stuck rigidly to Bruno's choice of components for my N Cores. He's got the test gear most of the smaller boys dont have - and knows how to use it.
I did find that minor tweaks to my speaker cabling allowed me to get the sound I wanted -which surprised me given the short speaker cables I have.
Im tri wired and used multistrand copper for bass and silver coated for mid and treble, all non directional.

Think about the size of the voice coils in the ATC's. They are not sensitive speakers and must generate a proportional back emf which you want to discharge through the low output impedance of the NCore. Make the send path and the return path impedance of the LS cables the same was my approach.

How have you connected up between Pre / NCores and ATC's

Im not a fan of the subjective terms used - and when you read the word subtle !
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Re: Op-amp Tweaking

Postby Ianmac » Sat Jul 14, 2018 7:19 am

On reflection,
I hope I havent gone OTT when talking about back emf .
For those newer members who dont understand, this analogy may help its - High School physics.

An electric motor rotates when you apply a voltage to it. The current passing through the windings of the motor generates a magnetic field which reacts with the static magnetic field within the motor and causes movement

A generator generates a voltage when you apply movement to it. The magnetic field within the generator induces a current into the windings resulting in a voltage being generated and a current made available.

Look at a Loudspeaker as a Motor. We pass a current through its voice coil from an amp and it moves in or out, dependant on current direction.

But its also a (small) Generator. As the voice coil moves in and out, the fixed magnetic field from the speaker magnet ensures a voltage is induced into the voice coil. We dont want this but its there and we do our best to get rid of it.
Its discharge path is the opposite to the driving current from the Amplifier and how the amplifier can dampen this back voltage (emf) is indicated by the amplifier output impedance (low as possible) or damping factor ( high as possible)

Its usually mentioned in the amplifier specification but you average Hi Fi sales person may not have a clue what you are talking about.
The response of speakers in a system can usually be improved by paying attention to the output impedance of the amp used to drive them and how they are wired.
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mojo
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Re: Op-amp Tweaking

Postby mojo » Sat Jul 14, 2018 10:43 am

Thanks Ian.

No, I'm not sure, and in fact having listened and swapped the £2 for the £60 op-amps a few times, I'm fairly sure the op-amp effect is below my threshold of detection. I've pretty much made up my mind not to spend £200 "upgrading" the op-amps to Sonic Imagery 994s - I know they are a small company and development costs will be high for such a low-volume item, but it's too much for me to try just in case it has an unexpected effect.

Speaker cables - yes, I agree. I'm currently running with some that I made up just to get started. They are short (but they could be shorter) and I've made them up with spades instead of bananas for better contact area at the connections. They are around 700mm long with a CSA of 5mm2 - which is Ok but I can do much better! I'm looking to triple these, and halve the length - which should be worth doing. The problem (that you will be familiar with, no doubt) is that as shipped from the factory, the Apollons have only one pair of terminals, and it's quite awkward to terminate multiple runs of fairly big wire onto one spade terminal! I'll probably solve that by putting two lengths onto one spade and a third length onto bananas - not perfect but probably good enough.

The other thing that's a compromise at the moment are the input cables, which are a pair of really cheap XLRs from Farnell (from memory I think around £3 each) so maybe I could spend a bit more there - but I'm not expecting that to have anything more than a very subtle effect (i.e. it too will probably be below my threshold of detection).

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Re: Op-amp Tweaking

Postby southpaw » Mon Jul 16, 2018 8:40 pm

Have you found any difference between rca / xlr inputs?

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Re: Op-amp Tweaking

Postby callen24 » Mon Jul 16, 2018 8:47 pm

mojo wrote:Thanks Ian.

No, I'm not sure, and in fact having listened and swapped the £2 for the £60 op-amps a few times, I'm fairly sure the op-amp effect is below my threshold of detection. I've pretty much made up my mind not to spend £200 "upgrading" the op-amps to Sonic Imagery 994s - I know they are a small company and development costs will be high for such a low-volume item, but it's too much for me to try just in case it has an unexpected effect.

Speaker cables - yes, I agree. I'm currently running with some that I made up just to get started. They are short (but they could be shorter) and I've made them up with spades instead of bananas for better contact area at the connections. They are around 700mm long with a CSA of 5mm2 - which is Ok but I can do much better! I'm looking to triple these, and halve the length - which should be worth doing. The problem (that you will be familiar with, no doubt) is that as shipped from the factory, the Apollons have only one pair of terminals, and it's quite awkward to terminate multiple runs of fairly big wire onto one spade terminal! I'll probably solve that by putting two lengths onto one spade and a third length onto bananas - not perfect but probably good enough.

The other thing that's a compromise at the moment are the input cables, which are a pair of really cheap XLRs from Farnell (from memory I think around £3 each) so maybe I could spend a bit more there - but I'm not expecting that to have anything more than a very subtle effect (i.e. it too will probably be below my threshold of detection).


do you need to quite long XLRs - i have van damme green with neutrik connectors to sell cheaply?
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Re: Op-amp Tweaking

Postby mojo » Mon Jul 16, 2018 9:02 pm

southpaw wrote:Have you found any difference between rca / xlr inputs?


Haven't tried, to be honest. I can't think of any reason why there should be any difference with the short lengths I am using, though.

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Re: Op-amp Tweaking

Postby mojo » Mon Jul 16, 2018 9:07 pm

callen24 wrote:do you need to quite long XLRs - i have van damme green with neutrik connectors to sell cheaply?


No - actually I only need them short. I think the current ones are 750mm, which is easily enough.

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Re: Op-amp Tweaking

Postby MiniCoupeman » Tue Jul 17, 2018 4:09 pm

I have put several long posts regarding Nord versions of your amp on this forum. When I first fired the Nord up the bass was overwhelming. Not nice. I read other feedback they all said - and they were right - it will settle down. Give it 100 hours. I honestly haven’t heard a better amplifier than mine and yes, it does keep getting better even afterv500 hours plus.

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Re: Op-amp Tweaking

Postby mojo » Tue Jul 17, 2018 4:56 pm

A large part of the decision to go with Apollon was based on the favourable feedback here!

It seems to be widely held that the Apollon is a "copy" of the Nord, but as I understand it the only bit of each that doesn't come direct from Hypex are the input buffer boards, and in both cases they seem to be based on a design published openly on the internet (by a Sonic Imagery employee) - so really it's just two versions of someone else's design. I've not looked in detail to see if there are significant differences between the Noord interpretation of that design and the Apollon one, but I doubt there can be any major differences.

I'm liking the Apollons a lot - can't say that I've notice the sound change significantly so far, but I'm probably at around the 50 hour stage at the moment.

Apologies if this is already covered elsewhere, but have you done any op-amp fiddling yet?

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Re: Op-amp Tweaking

Postby mojo » Wed Jul 18, 2018 2:35 pm

callen24 wrote:do you need to quite long XLRs - i have van damme green with neutrik connectors to sell cheaply?


How long is quite long, as a matter of interest? If they are a decent price I guess I could cut them down! If you think that's an option, please PM me a price and I'll get back to you. Thanks!