303 input impedance/volume pot

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16-Dec-2019 09:36 AM

SteveP

SteveP

Posts: 8

I'd like to work out the correct impedance for a new volume pot in my system. 

I am running two Quad 303 amps, Dada upgraded. The source is a laptop & DAC, so no preamp, just a passive volume pot I put in as a temporary solution a decade or two back. It works OK but doesn't take the volume down low enough, meaning I have to use the Windows audio stack to keep it in check. I'd prefer to bypass this and send the full signal out, unmolested by Microsoft engineers.

The numbers I know are:

2 x Quad 303 amp, input impedance of 22 ohms each (one for each L&R channel). I don't now if the Dada upgrade alters the imput impedance at all or whether using the amps like this effectively halves the 22 ohm figure?

DAC output impedance 5 or 50 ohms (I have two to choose from)

I'm looking at a choice of a 10k, 50k or 250k log potentiometer. Anyone good at maths?

Many thanks!



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17-Dec-2019 05:44 AM

EJP

EJP

Posts: 1690

Big error here. The input impedance of a Quad 303 is 20K, not 20 ohms, consisting of R100=220K in shunt and R101=22K in series, both seen as in parallel from the point of view of the source, as the base of Tr100 is a virtual earth. The Dada upgrades don't change this.

You need to replace R100 with say a 250K log pot. It doesn't matter which output impedance you choose on your DAC.

EJP

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17-Dec-2019 11:19 AM

PeteUK

PeteUK

Posts: 19

Are the 303's in series or parallel, are they wired according to the Quad documentation?
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17-Dec-2019 02:34 PM

SteveP

SteveP

Posts: 8

Many thanks, my mistake, 22k it is.

They are set up as one 303 amplifiing the left signal, the other the right, and the driver boards in each individually driving the high or low range of the speakers.

I've ordered a pot and will install it with a little more care than the old one. I've not had my system set up for a while and I'm loving how it sounds.

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17-Dec-2019 04:00 PM

PeteUK

PeteUK

Posts: 19

So you've got two 303's both in series, and you're going to fit 4 pots to replace the R100's in each channel? That's a lot of adjustment and I'd be concerned at what it would sound like tbh.

First of all have the left and right hand amps have been set to their null points, as in 'Quad 303 Monoblock Instructions' (In Dada downloads section)?

It makes sense to have everything in balance before going further.

 

 

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17-Dec-2019 05:55 PM

SteveP

SteveP

Posts: 8

Oh crikey, ignorance was bliss! I've just read the monoblock info and can see I cut a few corners. 

First confession: I've come to suspect what I fitted, back when Tony Blair was still the fresh-faced new PM, isn't even a volume pot, just a series potentiometer. I'd thought the way it was physically fitted, concealed in a scrap FM3 with the original tuning knob doing the turning, meant that I wasn't able to use the adjustment full range. It looks like I made a more fundamental error than that.

I was imagining that a new passive stereo volume pot, simply fitted in the signal path from the DAC (left and right channel RCA cables), would work as a volume control for the left 303 and right 303? Let me down gently if that's not so!

Also, now I'm wondering what to so with the parallel series wiring choice. The speakers are nominally 8ohms, but bi-wired, with one driver board in each 303 powering the high/mid and the other the low. Will the high/mid have a different impedance to the low? If 8ohms is the number, is there any advantage to one set up over the other?

Many thanks



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18-Dec-2019 03:20 AM

EJP

EJP

Posts: 1690

It is more likely to be a balancing potentiometer to equalize the channels, if they are already monoblocks.
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18-Dec-2019 11:24 AM

PeteUK

PeteUK

Posts: 19

Your easiest choice would be to use the parallel (ie no transformer required) arrangement. This will  be ok for your 8 ohm speakers.

I would start, as EJP says, by connecting a 5k pot as your balancing control, with the wiper connected to your signal source, and the balanced signal being fed from each end of the pot to each channel, as documented.

Of course any form of volume control would need to be prior to this arrangement, being controlled by your preamp, computer or other inline arrangement.

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18-Dec-2019 11:56 AM

SteveP

SteveP

Posts: 8

Many thanks, I'm going to have a look at how I wired them originally and will try a redesign after Christmas once the new pot arrives.

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18-Dec-2019 02:35 PM

PeteUK

PeteUK

Posts: 19

Good luck Steve, let us know how get on!
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15-Jan-2020 12:59 PM

SteveP

SteveP

Posts: 8

Well, I'm one step from nirvana. I was able to pick up a passive pot (Satgeline ILA-100RCA) which sits inline so the the chain goes: source-DAC-pot=303x2. It works perfectly.

I've realised the two 303s are arranged in a "vertically bi-amped" set-up and haven't done anything with them so far other than set the 67V/33.5V/10mV figures as per the service guide.

I've made a 2020 upgrade to the source material by going with a streaming system (Bluesound Vault) which is almost perfect except for its internal DAC. I have to stop listening after an hour whereas with my usual DAC and the same source files I'll listen all day. I didn't expect that! Anyway, have a new DAC (Chord Mojo) on order at which point I'm going to struggle to find reasons to leave the house.

Thanks for the help.

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