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Old 12th July 2013, 03:07 PM   #21
qusp is offline qusp  Australia
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Quote:
Originally Posted by thommy View Post
It's weiliang's latest 9018 board with the 4 opamps and 80 MHz clock
the ES9018 has one of the better volume controls available built in, you have to spend serious money to do better with analogue, really serious.

I dont feel like analyzing his latest dac, last one didnt have any proper source selection or access to the volume control, but if you use something like arduino/hifiduino, good luck matching the performance with anything that costs less than your entire dac does

forget all this other confusion, if your sources are digital, everything is connected to the dac and the dac is connected to your amps, forget trying to match it, just make sure whatever amps you end up using, dont have lots more gain than you actually need, then you are good to go, virtually free
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Old 12th July 2013, 03:13 PM   #22
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A digital volume control in the DAC can be calibrated and consistently repeatable. But the analog attenuator will "turn down" any background noise (hum, hiss, etc.) from the DAC along with the program material signal, helping keep consistent signal-to-noise ratio at low volumes.

When he says "connect the DAC output across the pot and the wiper to the amps" if the DAC is single-ended output referenced to a signal ground, ground the "other" end of the pot. The DAC will primarily see the same resistance across the pot always, and the high impedance of the power amp input won't change the DAC's behavior much as you rotate the knob. DAC signal on the terminal which the internal wiper points to when the knob is turned full-clockwise (viewed from the back, with terminals up, that's the terminal on the right), ground on the opposite (left) terminal, and the middle terminal (wiper) to the power amps. A dual-ganged pot is convenient, but then you may want another special-purpose 'balance' pot. Of course, two seperate pots are less convenient because you always have to adjust their relative levels. You will have no "loudness" correction for the Fletcher-Munson correction curve (our ears hear bass & treble worse at low volumes) with just a simple pot. If there's DC on the DAC output make sure the power amp input is capacitor-coupled or add a cap to each.

Because this is so extremely simple, it has the potential to sound very good. If it doesn't, and needs frequency-responce 'tone' adjustment, depending on your digital input format a good outboard line-level digital DSP EQ is always a possibility if you need it. If it's all in one box with speakers, besides in the "preamp" you can fine-tune the tone of the power amp to match the speakers by adjusting the inductance and capacitance in the interstage coupling or in the feedback loop or even via speaker-level filters.
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Old 12th July 2013, 04:04 PM   #23
qusp is offline qusp  Australia
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I take it youve never used the es9018 volume control?

he wants a simple solution for volume control, why does everyone need to make this more complicated than it is?

if hes bought this chinese version of a 9018 dac, i'm guessing there isnt that much budget here

just use the es9018 volume control, its excellent, i've used nothing but it for years now and I have experience with high quality elma steppers, alps blue, relay based etc. I have several friends who have done the same, as has opc and just about everyone ive turned onto using the one in the 9018, these are people using F5X poweramps, STAX o2 MkII headphones with DIY T2 electrostatic amps which previously were running black beauty's, 4 deck zfoil steppers. they, like me, dont even build pots into their designs now, I dont do it to save money and neither do they.

its effectively free and its extremely hard to match with an analogue control. when you have a DAC with SNR/DNR/THD like 9018, unless youve done something really dumb with your gain structure, you are just spending money and time for no reason at all using anything else.

I use it driving owen's poweramps, as well as driving JH13 in ear custom stage monitors with 'the wire' headphone amps, which are pretty much as transparent as you'll get and the jh13 quite low impedance, SNR is not a problem.

Last edited by qusp; 12th July 2013 at 04:10 PM.
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Old 12th July 2013, 05:18 PM   #24
thommy is offline thommy  United Kingdom
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Quote:
Originally Posted by qusp View Post
I take it youve never used the es9018 volume control?

he wants a simple solution for volume control, why does everyone need to make this more complicated than it is?

if hes bought this chinese version of a 9018 dac, i'm guessing there isnt that much budget here

just use the es9018 volume control, its excellent, i've used nothing but it for years now and I have experience with high quality elma steppers, alps blue, relay based etc. I have several friends who have done the same, as has opc and just about everyone ive turned onto using the one in the 9018, these are people using F5X poweramps, STAX o2 MkII headphones with DIY T2 electrostatic amps which previously were running black beauty's, 4 deck zfoil steppers. they, like me, dont even build pots into their designs now, I dont do it to save money and neither do they.

its effectively free and its extremely hard to match with an analogue control. when you have a DAC with SNR/DNR/THD like 9018, unless youve done something really dumb with your gain structure, you are just spending money and time for no reason at all using anything else.

I use it driving owen's poweramps, as well as driving JH13 in ear custom stage monitors with 'the wire' headphone amps, which are pretty much as transparent as you'll get and the jh13 quite low impedance, SNR is not a problem.
I understand and thanks for explaining it but I really would prefer an analogue volume knob, particularly if I choose to include a tube pre. I made a post elsewhere on this forum regarding passive summing, as it would make sense to include an analogue input to make the unit more versatile so controlling volume from the dac limits my options somewhat.

My budget is about 400GBP, so I should be able to include a decent quality stepped attenuators in there.
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Old 12th July 2013, 05:28 PM   #25
thommy is offline thommy  United Kingdom
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So to summarise what the other have said:

1. It's quite feasible to connect the dac or summing circuit directly to the amps via a 100k stepped attenuators.

2. The low impedance of the amps may result in a lack of gain at the input and this will adversely effect volume and therefore efficiency (?).

3. The use of a tube pre between the source(s) and volume attenuators would include a gain circuit and therefore, with modification, offers the potential to compensate for any resulting gain discrepancy caused due to the low input impedance at the amps.

My question now, is how do I know when the input gain is correct? Can I measure the signal coming from the attenuators (fully open) and reference that against what the amp is expecting to receive?
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Old 12th July 2013, 08:01 PM   #26
AndrewT is offline AndrewT  Scotland
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A dual track log law 20k potentiometer can cost from $5.
Use that to check if you need to spend more money.
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Old 12th July 2013, 08:20 PM   #27
M Gregg is offline M Gregg  United Kingdom
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NB,

I don't like stepped attenuators its one click to much or one not enough.

But some like them, I prefer a good log pot.
Do you really want a stepped attenuator?

Don't bother measuring just try the pot and connect it. Try it see what it sounds like if it is loud enough etc. then measure.
I bet the DAC will drive it direct, however the tube section will give you a different animal that you might prefer.
If you have more gain than you need it will drive the power amps to full volume before the full pot rotation, that might be better if you have some quiet tracks or inputs.
What you don't want is full power just off the start of the pot rotation, however full power at about 2/3 of the rotation is OK.
You want to be able to have some usable volume at low level at the start of rotation for night listening. Nothing worse than to loud just above the 0 position.

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Last edited by M Gregg; 12th July 2013 at 08:39 PM.
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Old 12th July 2013, 08:59 PM   #28
thommy is offline thommy  United Kingdom
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My preference would be amps at 95% power upon full pot rotation. I definitely don't want full power near the start with a pair of 250w amps.

This little project is going to become the sound system in a bar so needs to cope with being turned up all the way when people want to get lively, and I won't generally be there to turn the knob back down.

A stepped volume control would skew the signal less than a dual track pot and with 23 positions there's bound to be one that's suitable, as its going up in notches of about 10w (I assume that's why we use log pots - as the dB scale is logarithmic, using a log pot would give a linear rise to audible volume level?). I understand what you mean about always wanting the setting in between two notches but the steppers do sound better, and a little tweak of the source volume on the computer could sort that niggle out.

You mention 'loud enough', but how do I know when the amp is being given a signal that is allowing it to run at 100%?

Surely the only way to do that is to measure the output voltage from the attenuator, and compare that to the maximum input voltage allowed for the amp module.

Andrew, you mention 20k, is that for a particular reason? How would the impedance value of the volume control affect the output? Does changing the pot from 20k to 100k affect the maximum output level?
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Old 13th July 2013, 06:30 AM   #29
AndrewT is offline AndrewT  Scotland
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20k chosen as a sensible load impedance for the DAC and a not too high output impedance to feed the power amps.
The maximum output impedance of a 100ohm Source combined with a 20k vol pot is 1/4*[100+20000] = 5025ohms.
This needs a short low capacitance cable between the vol pot and the power amps.
10k will allow slightly longer cables, but you are still limited to about 2m to 3m.

The gain (output voltage) is not affected by the vol pot resistance.

For a PA system with potentially long cable runs and high interference (coolers and motors in and around the bar) needs much more thought than you seem able to give.
Go to someone who has experience in PA duty.
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Old 14th July 2013, 08:58 PM   #30
thommy is offline thommy  United Kingdom
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The total length of signal cable from DAC to amps will be <50cm as the whole thing is in one box. The DAC outputs 2VRMS, so a 20k pot will be low enough to match the input impedance.

Andrew, I appreciate your comments about the PA stuff although don't think that will be such an issue in this case, and for your patience with my obvious lack of understanding. If from what I've written you believe that 20k is the best choice for this particular application then i say thankyou very much, and will get a 20k dual track log pot to try it out as you suggested. If that works and sounds ok i'll just leave it at that.
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