34 compatibility

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17-May-2018 01:24 PM

Martin

Martin

Posts: 5

I have been using a 34/405-2 set up for many years into various "BBC" loudspeakers with complete satisfaction. I recently bought an Quad Artera power amp and have modified the 34 output to give me 1v to drive the Artera, and frankly it sounds awful. The bass is missing and there is a spike somewhere around 7kHz which makes it overly bright and aggressive sounding. The frequency response is all over the place. I have come to the unconfirmed conclusion that the fundamental difference is the input impedance of the power amp, the 405 is 100k and the Artera 15k, my question, is the significantly higher load now on the 34 output affecting it's frequency response and can I do anything about it? Does this sound plausible or could it be something else altogether. Thanks Martin.

 

 

 

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19-May-2018 04:23 AM

EJP

EJP

Posts: 1525

That's not correct. The input impedance of a Quad 405 is specified in the User Manual as 20K in parallel with 220pF. Your problem lies elsewhere.

EJP
Dada Electronics Australia

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19-May-2018 10:32 PM

Martin

Martin

Posts: 5

Indeed so and I can't now remember where I got the figure of 100k from, surely I didn't make it up. The circuit diagran shows the very first component on the input to be a 220k resistor to ground which I think forms the basis of the input impedance, have I grossly misunderstood this, entirely possible?

Somewhat more bizarre when playing some stuff tonight the sound has become considerably more natural and whilst I would subjectively suggest the Artera is brighter than a 405, it seems to be settling down. How very strange. I'll see how this develops.

Regards

Martin

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20-May-2018 04:48 AM

EJP

EJP

Posts: 1525

The input impedance is mostly defined by the 22K input series resistor, remembering that it is looking into an inverting input, which is a virtual earth, so it is a shunt to ground. The 220K resistor is there to discharge the input capacitor. It is also in shunt to ground. So you have 22K || 220K = 20K. The 220pF must come from the op-amp, and if you modernize it it is probably quite a bit lower.

EJP


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20-May-2018 03:01 PM

Martin

Martin

Posts: 5

Thanks very much for that explanation EJP, I really don't know where I got 100k from. On that basis since there is a difference but not major between the 405 and the Artera the problem, as you say, has to be somewhere else. Regards

Martin


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22-May-2018 07:00 PM

Martin

Martin

Posts: 5

Well after a swap of emails between IAG and me it seems likely the 34 has a fault, which is no bottom end, I imagine this will be in the circuitry around the filters as it affects all inputs. Since I have no access to test equipment these days it's probably going to be more cost effective to get IAG to service it. Any thoughts about this? Regards Martin
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23-May-2018 03:12 AM

EJP

EJP

Posts: 1525

Well if that's the case I would start with R47/48, but I don't follow this. You said you had been using the 34/405-2 combination with complete satisfaction. I don't see how that can be squared with a fault in the 34. Or is it no longer the case? I think you were on stronger ground in the first place. I would start by replacing C77/78, with non-polars, or orange-drops if you can get them.


I should also have asked how you modified it to get 1V output. You have to preserve the sum of R117+R118+R119, or increase it, rather than decrease it, and similarly in the other channel.
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23-May-2018 05:58 PM

Martin

Martin

Posts: 5

Hi EJP

I should explain more fully, the 34/405 was in use until 14 months ago driving LS3/5s, and I certainly assumed it was all working fine. They were all put into store when I the was given my fathers 77 equipment and a pair of KEF Reference 1s. Recent circumstances allowed me to upgrade the power amp to the Artera and use the 34 as the pre amp, when the deterioration of the freq response became apparent. I assumed that changing the output level of the 34 to adequately drive the Artera had some impact, IAG have confirmed that this is not the case. I therefore backtracked the 34 to 0.5v output and substituted the 405 for the Artera, with the same lack of bass. The common components are the 34 and the LS, of which a faulty 34 is the most likely. It is 26 years old so I would expect the caps to have aged, quite possibly to the point where there is an audible effect, that's my take on the issue. The mods to the 34 were done following the advice on the site. I'll have a close look at it and decide what I should do.

Martin

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26-May-2018 06:52 AM

EJP

EJP

Posts: 1525

If it's in both channels it can really only be the PSU or else your output level mods, if you managed to make the same mistake in both channels, which isn't likely.

Don't overlook the LS: if the rubber surrounds have deteriotated it could cause exactly this. Mine disintegrated at 30 years after about 15 of non-use.

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