Cyrus 2 Resistors

[Sonusthree]

Much obliged Geoff. You've been a great help.

I've hooked up the Cyrus 1 to the tweeters on my speakers in partnership with a Cyrus 2 and PSX for the rest. It is definitely imparting some of it's magic but I'll have to find a better solution. I wish I had 3 way speakers instead of 2.5 way.
I didn't know how much of the 'performance' I was missing. I always had faith that higher resolution would bring me closer to the music eventually and this is a very positive step forward.

Anyway, Thanks for the schematic Geoff. Hopefully this will be my first proper amplifier fix. My girlfriend laughs at the amount of our friends electrical equipment that I have fixed with cotton buds, post-it-notes, switch cleaner and grease.

Martin.

[AndrewT]

Hi Sonus,
I suspect Cyrus had the gain in the power amp sections exactly matching as they moved through the various versions.

How have you arranged the gain in the bi-amped arrangement?

BTW,
I have been recommending bi-amping for years, even to the extent of using different amps for the different frequency ranges.
Your treble off the One and bass/mid off the Two+PSX seems to be the way to go.
Has that got rid of the dullness?

Is the dullness due to the Two using parallel pairs, or due to adding the PSX or due to modifying the PSX?
Could the last two causes be "cured" by adding decoupling inside the Two case? Big film caps right on the PCB?

[Geoff]

Originally posted by AndrewT
I suspect Cyrus had the gain in the power amp sections exactly matching as they moved through the various versions.

The Cyrus 2 has a higher closed loop gain than the Cyrus 1 in both v6 and v7.

Is the dullness due to the Two using parallel pairs, .......

Martin has indicated that he is using a v7 Cyrus 2. This does not have parallel output transistors. It uses the larger PT77 (BUV48) instead of the PT7 (BUV28) fitted in the Cyrus 1.

[AndrewT]

Hi Geoff,
can you explain the dullness?

How does one bi-amp if the gains in the power amps are different? I know what I would do, but my solution may not be the easy way.

[Sonusthree]

Originally posted by AndrewT
Hi Sonus,
I suspect Cyrus had the gain in the power amp sections exactly matching as they moved through the various versions.

How have you arranged the gain in the bi-amped arrangement?

BTW,
I have been recommending bi-amping for years, even to the extent of using different amps for the different frequency ranges.
Your treble off the One and bass/mid off the Two+PSX seems to be the way to go.
Has that got rid of the dullness?

Is the dullness due to the Two using parallel pairs, or due to adding the PSX or due to modifying the PSX?
Could the last two causes be "cured" by adding decoupling inside the Two case? Big film caps right on the PCB?

Hi Andrew,
I'm using issue 7 (fully metal/toggle switch) amps and both appear to be almost identical except for the power supplies, transistors and any mods I have made. I was going to start a new thread for this today because there are a lot of interesting questions raised here. I'm running both amps discretely with the Two being fed from the 'tape out' of the One and so they have independent volume controls. I thought that this may be problematic but have found it very easy to get the levels matched. There is only one setting that works. It is slightly problematic that the Cyrus One has the original 'clicky' volume pot and the correct volume setting is always between clicks. I intend to convert them both to power amps and have an outboard passive driving both. I'm also quite interested in trying a stepped attenuator.

I am trying to work out my options here. The Cyrus One (totally stock) is working the Tannoy 611 above 2.5KHz and the difference is absolutely worthwhile and exciting. I need to find a way to bring more resolution below 2.5KHZ but I am limited by the 2.5 way configuration.

The best way forward, I think, is to investigate all those pesky time constants that you told me about so many times and try some proper bypassing and snubbing on the C2/PSX. I was looking for more bass but I have obviously sacrificed something in the process. Hearing the stock Cyrus One makes this blindingly obvious.

Main mods to Cyrus 2/PSX:
30,000uF slit foil changed to 88,000uF Mundorf M-Lytic.
High speed diodes.
All electrolytics replaced.
Input cap raised from 2.2uF electrolytic to 3uF Sonicap (polypropylene).
Alps Blue volume pot (balance removed).

Main questions for me:
Are the slit foils in the Cyrus One special after all or is it just that they have less capacity?
Are the differing transitors significant?
Does larger transformer change only the bass for the better?
Was the higher value input cap a bad idea?

I'm going to try 100nF caps across the PSX diodes but do you have any recommended values for snubbers/bypassing?

Martin.

[Sonusthree]

Originally posted by AndrewT
Hi Geoff,
can you explain the dullness?

How does one bi-amp if the gains in the power amps are different? I know what I would do, but my solution may not be the easy way.

I hate to admit it but I can't think of any other way except to change the negative feedback. I am very wary of doing this partly because I don't really have the expertise and partly because I have recently mucked about with the feedback in the Cyrus 2 and the input filter. My opinion, as a novice, is that it is a very delicate balancing act. I now realise that these amps are cleverly designed to squeeze everything from the design. I have gained alot of admiration for the designer and trust their choices a lot more and perhaps even an electrolytic as the input cap is for a reason?

Martin.

[AndrewT]

Hi,
changing the global feedback to alter the gain also changes the phase and gain margins.
This requires both expertise and instrumentation to get right.

Are there a pre out, power in set of RCAs on the Two? takes the pot and pre circuitry out of the equation.
I would adjust the Rs/Zin ratios to match the power amp overall gains (i.e. fixed attenuator) but it can alter output offset and HP filter.

I wonder if the dullness would show up as a frequency response difference. If it did, it would be easy to measure/modify/listen/measure/modify/listen etc.
But, I suspect that the dullness will not show as a response error. That takes the measure out and it becomes modify/listen/modify/listen and maybe never get to a solution.

[Sonusthree]

Hi,
It's a shame that I don't have a scope. I use my soundcard for line level work. The Two doesn't have pre outs so I was thinking of making my own pre to feed them both just as they are now with a high pass filter preceeding the pot/stepped attenuator. I will try to keep the wires as short as possible of course and possibly add a switchable buffer just for experimentation.

Do you think I can recover the upper mid range detail from the Two? It only has to work well up to 2.5KHz. I know that you are a specialist in this area. Can you give me some ball park suggestions for a snubber or more local decoupling? I'll be reading up on it tonight though but I know that you guys understand the interactions of LCR a lot more than I do.

Many thanks,
Martin.

[Geoff]

Originally posted by AndrewT
Hi Geoff,
can you explain the dullness?

How does one bi-amp if the gains in the power amps are different? I know what I would do, but my solution may not be the easy way.

I would never describe the unmodified Cyrus 2 + PSX as 'dull'. When I did a side-by-side comparison between this combination and a Cyrus 1 in my small, at the time, listening room and with my preferred music (acoustic Jazz and Baroque classical) I found the 2+PSX sounded a little 'heavy' compared with the 'delicacy' of the 1 and on balance I came out in favour of the latter. I suspect that the difference between the two is primarily down to the power supply rather than the different output transistors though the latter could contribute.

I have only bi-amped using two Cyrus 1s (with both passive and active filters) so the gain difference question did not arise. However, I see no problems in changing one of the feedback resistors in the Cyrus 2 (750R) to match the value used in the Cyrus 1 (1k), or vice versa, so that the gains are equal. The same pcb, schematic, compensation components etc are used for both amps so there should be no stability problems.

[Geoff]

Originally posted by Sonusthree

Main questions for me:
Are the slit foils in the Cyrus One special after all or is it just that they have less capacity?
Are the differing transitors significant?
Does larger transformer change only the bass for the better?
Was the higher value input cap a bad idea?

At the time they were introduced, the slit foil capacitors did gain a reputation for sounding better than normal electrolytics of a similar value.

I doubt that the different output transistors are having any significant effect, nor the increased capacitance of the input capacitor. The type of capacitor used at the input will have a noticeable effect as will the fact that you have changed to a conductive plastic volume potentiometer. In conjunction with the changes made to the power supply, the input capacitor and potentiometer may well be robbing your Cyrus 2 of some of its 'life'.

Have you tried disconnecting the PSX and replacing the rail fuses in the amp so that it runs off its internal (smaller) transformer and slit foil capacitors? This may bring its performance closer to that of the Cyrus 1 as regards resolution but you may lose out elsewhere in the bass. It could at least narrow down the possible culprit(s) to either the modified PSX, if there is an improvement, or the input capacitor or potentiometer if there isn't.