The *NOT BORING!* Hi-Fi faff chat and sales

So it continues...

A brace of good quality OPA134 op-amps to replace the not so hot (though state of the 'fet input' op amp art of the time) TL071s in the pre and power amp - a starter for ten. Also some decent speaker connectors to replace the woeful original spring push ones. A mod also done by Quad to later 405's.

DSCF0214_zps5accac27.jpg


Got the pre-hooked up to my 'phones this eve. Bass is super tuneful, treble nice and sweet and easy - bass definitely easier to follow than the Arcam. Mid... jury out. Slightly 'thick' sounding maybe? Maybe lacking in detail? I know the op-amps will reduce the noise level, but detail? Who knows... one step at a time.

Pleased with the effort that's gone into the power supplies on the pre, will leave that alone aside from faster rectifiers. Still not convinced that I need a buffer on the CD input - may add a 'straight through' switch to the 'Aux' card, with a bit of attenuation - one less active stage must be a good thing. Input switching, uses CMOS 4066's from eons ago, which apparently add 0.01% distortion all by themselves. (I designed a pre back in about '85 and thought about using them, but gut feeling led me to abandon the idea and I used CMOS latches to drive telecoms relays instead) Yes, 0.01% is low, very low, but it can be bettered by later versions of the 4066 so I will probably look there. Torn though as ideally I would fit reed relays rather than relay on CMOS (spit!) to do my switching but I dont want to make it less 'Quad' so I'll try Maxim's versons of the 4066 which have huge bandwidth an should be almost an order of magnitude better THD-wise in a low signal level switching application, which generally means other aspects of performance will be better also.

It's never going to be the most transparent of pre's, but it is a lovely piece of design work. A joy to work on and to use. The power amp however, that has the potential to be truly phenomenal. A very clever design, and something I'm sure will respond to reworking with a few modern ideas and components applied sensitively. Arise FrankenQuad! :twisted:

<edit> listening to some old Rage Against the Machine - bass guitar is about as 'real' as I've heard on anything. Yes, the low end certainly does have some serious dynamics!
 
When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Re: The Hi-Fi Chat and sales thread

The premise is bullshit, but at least you get some stuff in the box ;)

The blurb about those caps is nonsense. If anything, the ones that he dismisses are better (more conductive), if you could imagine that there was a mechanism to cause problems in the first place that is! I just couldn't bring myself to sell stuff like that - wouldn't be able to sleep at night! :LOL:
 
Re: The Hi-Fi Chat and sales thread

Back with another question ;)

One of the 'is this any good' variety:
Sherwood RD6103R amp

Likely to be surplus very soon and wondered if it was worth owt or if it was any use to anyone here? Cost me £60, not that that means anything really! :LOL:
 
When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Re: The Hi-Fi Chat and sales thread

For those of you who enjoy this sort of thing, I have been working on the AUX (CD) input card again this evening. I've replaced the TL071 op-amps with the OPA134's, reduced the gain a little more by changing the input voltage divider to approx 10:1 (which is a wee bit low now!).

This the original Quad AUX card

auxunmod_zpsad790bf1.jpg


and this is as mine stands now

auxmod_zps33c364f0.jpg


You can see the op-amp is configured as a unity gain buffer (all of the output returned to the inverting input) which has high input impedance (Z) and low output Z. I think this may impact the cutoff frequency of the output coupling, so the next step may be to remove the input and output coupling caps as there should be no DC at the output of the opamp. However, this begs the question "why bother with the buffer at all?" as my CD player has low output impedance anyway. Therefore I will probably end up with the below!

auxatten_zps26117f0a.jpg


This is simply a 10:1 attenuator to reduce the level from the CD player to suit the sensitivity of the preamp - though something like 6:1 (change R600/601 to 330k) will probably be more appropriate.

Subjectively, it sounds perhaps a bit 'faster' (which could be a subtle roll off of VLF which removing the coupling caps will reveal) and more forward.
 
Re: The Hi-Fi Chat and sales thread

There it is! :D

Changed the rest of the op-amps on the main board, did some local decoupling of supplies to op-amps, hand matched a load of metal film resistors in the signal path, replaced capacitors in the signal path, and bypassed the caps on the aux card because (as I suspected) there is negligible DC offset.

auxmod2_zps219e0061.jpg


I knew there was transparency in there waiting to get out. Deep bass restored, speed undiminished. Imaging is becoming three dimensional where it was always wide and defined. Oh yeah! The pre is really starting to rock! Looking forward to getting that power amp to the best it can be, as it already slays the Arcam which sounded restrained in comparison.
 
Re: The Hi-Fi Chat and sales thread

2 x AD8610 opamps for sale, taken out of a new headphone amplifier, so used for a few hours only....pics later, soldered in an 8 pin carrier if thats the right word. £10 posted
ooo008_zpsb3f6577a.jpg
 
Back
Top