To all the audio guys out there. - Page 1
Forum section: electronics / testgear

author: pope
author_rank: Frequent Contributor
timestamp: November 12, 2024, 10:15:32 pm
content: I have a QuantAsylum QA403 and although I like it quite a lot, I'm having hard time to measure the maximum level and headroom of pro audio DUTs (compressors, EQs, mic pres, etc...). The QA403 will output "only" around 20dBu which won't cut it for such measurments.So, the idea is to use some sort of an amplifier between the QA403 and the DUT. It has to be balanced I/O with low THD and noisefloor. Bandwidth isn't too critical I suppose. Anything above 40kHz would do.Has anyone run into a similar situation and if so, how did you resolved it?

author: KungFuJosh
author_rank: Super Contributor
timestamp: November 12, 2024, 10:25:26 pm
content: I want one of those, but don't have it yet. Have you tried their forum?https://forum.quantasylum.com/

author: pope
author_rank: Frequent Contributor
timestamp: November 12, 2024, 10:27:30 pm
content: Quote from: KungFuJosh on November 12, 2024, 10:25:26 pmNot yet. I though I'd try my luck here first

author: bson
author_rank: Unknown
timestamp: November 12, 2024, 10:33:53 pm
content: Input and output signal levels? What is the input impedance of the DUT? Unless it's too demanding this should be easy to whip up yourself with one or two fully differential opamps.

author: KungFuJosh
author_rank: Super Contributor
timestamp: November 12, 2024, 10:37:27 pm
content: Quote from: pope on November 12, 2024, 10:27:30 pmSure, but here you'll get EE answers to an audio question that I'm sure has been asked there before. If you do get an answer from there, please share here. 😉

author: pope
author_rank: Frequent Contributor
timestamp: November 12, 2024, 10:39:52 pm
content: Quote from: bson on November 12, 2024, 10:33:53 pmNot sure I follow. "Input and output signal levels" of which device?Input impedance of the DUT will vary depending on the DUT. Anything above 600Ω I'd say.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: November 12, 2024, 10:41:25 pm
content: Quote from: pope on November 12, 2024, 10:15:32 pmBy making this:and this[ Specified attachment is not available ]

author: Messtechniker
author_rank: Frequent Contributor
timestamp: November 12, 2024, 10:42:24 pm
content: and this power supply

author: KungFuJosh
author_rank: Super Contributor
timestamp: November 12, 2024, 10:44:31 pm
content: But then the Analyzer is profiling those things also...

author: bson
author_rank: Unknown
timestamp: November 12, 2024, 10:46:19 pm
content: Quote from: pope on November 12, 2024, 10:39:52 pmOf the amplifier you need!Amplifying 5 to 50V is very different from amplifying 0.5V to 5V even though the gain is the same.The DUT input impedance is important because it determines the power you need on the amplifier output (apart from the signal level).Edit: spelling it out

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2024, 10:46:50 pm
content: Profile the additional gadget, then de-convolve its properties from the overall properties with gadget and device-under-test.

author: pope
author_rank: Frequent Contributor
timestamp: November 12, 2024, 10:58:12 pm
content: Quote from: bson on November 12, 2024, 10:46:19 pmI would probably need a maximum of 10db of gain so something like 7Vrms to 24Vrms I supposeQuoteWell, the point is to be able to test various DUTs, isn't it?Pro audio gear Input impedance can range from 200Ohm all the way to 1Mohm with 600, 10K and 100K being some of the most common values.Ideally, I was hoping to find an off the shelf solution.

author: KungFuJosh
author_rank: Super Contributor
timestamp: November 12, 2024, 11:02:53 pm
content: Quote from: pope on November 12, 2024, 10:58:12 pm

author: Messtechniker
author_rank: Frequent Contributor
timestamp: November 13, 2024, 03:58:11 pm
content: Because I could not find such a thing, I built one.

author: pope
author_rank: Frequent Contributor
timestamp: November 13, 2024, 04:13:08 pm
content: Quote from: Messtechniker on November 13, 2024, 03:58:11 pmWhat are you using it with? QA403?

author: Messtechniker
author_rank: Frequent Contributor
timestamp: November 13, 2024, 07:58:14 pm
content: M-Audio Delta AP 192 and Audio Tester software.And Siglent 2042X

author: David Hess
author_rank: Super Contributor
timestamp: November 14, 2024, 01:58:09 am
content: Quote from: pope on November 12, 2024, 10:15:32 pm20 dBu is 22 volts peak-to-peak. so the QA403 is probably relying on integrated operational amplifiers with a 30 volt supply. This matters because it means an external amplifier will have to use a higher voltage design making it non-trivial.QuoteI have, and I immediately designed and built a small amplifier.Unfortunately high voltage operational amplifiers are not usually suitable for low distortion audio, so some kind of hybrid design will probably be required.

author: Someone
author_rank: Super Contributor
timestamp: November 14, 2024, 06:20:13 am
content: Quote from: David Hess on November 14, 2024, 01:58:09 am30dBu/25Vrms/70Vp-p is achievable for a differential signal (as pro audio tends to) on 36V rails so those levels aren't out of this world or impossible to find in commercial products. However pro audio does get off into crazy prices for what it is.

author: Zero999
author_rank: Super Contributor
timestamp: November 14, 2024, 11:14:13 am
content: Quote from: Messtechniker on November 12, 2024, 10:41:25 pmIt appears as though you either forgot to upload the second attachment, or the forum lost it.Regarding the first attachement. To change it from 125V to 48V, R4 and R10 need to be decreased from 47k to 18k, otherwise T1 and T6 will be off.

author: David Hess
author_rank: Super Contributor
timestamp: November 14, 2024, 01:47:16 pm
content: Quote from: Someone on November 14, 2024, 06:20:13 amThe specifications say they are using +/-12.5 volt supplies to generate a 22 volt peak-to-peak output (single ended) with 1.5 volts of headroom. Modern 40 volt audio operational amplifiers running on +/-18 volt supplies under the same conditions could produce 33 volts peak-to-peak single ended, or 66 volts peak-to-peak differential, although driving even a 10 kilohm differential load would be questionable. I am not sure that is a worthwhile improvement, and the limited output current may not be enough. That leads me to a hybrid design with a discrete output to get enough performance with a much greater output level.I would probably start with the design shown below, doubled and modified to make a differential amplifier, however I do not know anything about its distortion performance. There are plenty of examples online of boosted operational amplifier designs.

author: squadchannel
author_rank: Frequent Contributor
timestamp: November 14, 2024, 02:03:43 pm
content: Quote from: AVGresponding on November 14, 2024, 10:04:49 amMark's videos are always interesting.The comments also makes me laugh.Amplifier design, well $25,000 is ....It's ok, it's for audiophiles.

author: Someone
author_rank: Super Contributor
timestamp: November 14, 2024, 09:36:40 pm
content: Quote from: David Hess on November 14, 2024, 01:47:16 pmYou said (and I quoted) that higher than 30V is non-trivial. Yet there are numerous high current drive opamps with 36V supply rails (abs max 40V or higher) that can drive 10k easily, supported by both data sheet typical characteristics and real world uses. The 600Ω request from the OP is a little harder but paralleling operational amplifier outputs is considered pretty trivial in the audio world. Again, a quick perusal of some candidate parts readily finds solutions.Not hard at all, while sticking with well defined distortion and dynamic performance.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: November 15, 2024, 12:16:17 pm
content: Hello pope,Quote from: Messtechniker on November 13, 2024, 03:58:11 pmMaybe using a high voltageopampwith power output (200 mA) ?For the power supply, two 30 VA 2 x 24 VAC toroidal transformers + rectifiers + heavy filtering + fuses :https://www.amazon.fr/dp/B00MWRLX8Wwill lead at least to ± 75 VDC rails (low output load on a small transformer leads to +15 % output voltage).Signal peak voltage will probably be able to scrape 65 Vp (35,5 dBu). With this power opamp, asking for 20-26 dB of gain is not a problem even with tens of kHz of bandwidth.Put the opamp on a huge heatsink. Keep all wires/tracks as short as possible.PS : it would be so nice if the title of this thread reflects what we are discussing about

author: trobbins
author_rank: Frequent Contributor
timestamp: November 15, 2024, 12:49:09 pm
content: The OP appears to just want to measure headroom and simple signal stuff, not harmonics down to -120dB, so perhaps any form of battery powered linear amp with sufficient output signal level would seem fine - including an opamp, assuming it can drive the anticipated lowest loads at the max signal. May need to use a few 12V batteries, or 9V batteries, and an opamp with say 30V or +/-15V supply, or multiple of batteries. I have an LM1875 board for that purpose, with a suitable gain to just not clip with available level from my soundcard, but there would be many other simple boards on ebay etc.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: November 15, 2024, 01:44:16 pm
content: The audio analyser pope owns already outputs + 20 dBu (11 Vpeak) which can be generated by using ± 15 VDC rails.As pope needs signals with higher values, a low distortion PA (<> Class C) needs voltage rails above the 15 VDC mark.With ± 22 VDC (max value for most opamps), 17 Vpeak (24 dBu) can be obtained, best case. That is just 4 dB more that the analyser.For more output voltage, amplifiers that accept higher voltage rails are required.By pushing the LM1875 that trobbins suggested to its' limits (so 60 VDC or ± 30 VDC supply), 24 Vpeak (27 dBu) should be possible : a 7dB improvement.But pushing a PA to its' limits always raises distortion.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: November 15, 2024, 07:09:57 pm
content: I also built this one using an OPA 551 PA

author: djsb
author_rank: Super Contributor
timestamp: November 15, 2024, 07:20:37 pm
content: They have a transducer driverhttps://quantasylum.com/collections/frontpage/products/qa461-transducer-driverThe forum is probably the place to ask about your requirementshttps://forum.quantasylum.com/I have a QA403 but have not had a chance to use it yet. It's a nice piece of kit.

author: David Hess
author_rank: Super Contributor
timestamp: November 15, 2024, 08:09:18 pm
content: That OPA551/OPA552 is a good option for the simplest design with good performance. I wonder if a hybrid design using something like an OPA1611 could do better. Noise would be 1/3rd, and distortion potentially up to -136dB instead of -106dB, which I think points out the problem of combining the best audio performance in a high voltage and current part.It is difficult to evaluate the LM1875 because the datasheet only discusses performance with 4 and 8 ohm loads. I have used parts like that within the feedback loop of a better amplifier for lower distortion and noise, but the frequency compensation can be difficult. Better is to use a power current feedback amplifier, but I do not know of any with higher voltage ratings, leaving discrete designs like the one I posted.

author: The Soulman
author_rank: Super Contributor
timestamp: November 15, 2024, 08:19:53 pm
content: Easier to implement is a dedicated line-driver ic such as:https://thatcorp.com/that-1606-1646-balanced-line-driver-ics/

author: pope
author_rank: Frequent Contributor
timestamp: November 15, 2024, 08:44:11 pm
content: Thank you all for the replies. A lot of valuable information to digest once I find some time.I have some 1646s laying around.I hadn't thought of the qa461 but it seems to be unbalanced, right?

author: trobbins
author_rank: Frequent Contributor
timestamp: November 15, 2024, 10:24:00 pm
content: Another option is an audio signal step up transformer. Not sure about your software's capabilities, but with REW you could do a loopback calibration curve to normalise the amplitude response, and if you really wanted to lower harmonic levels of your test setup then you could null the loop harmonics out by modifying the harmonic levels/phases of the signal generator.

author: The Soulman
author_rank: Super Contributor
timestamp: November 15, 2024, 11:22:58 pm
content: Quote from: pope on November 15, 2024, 08:44:11 pmI couldn't find much if any specifications on the qa461 apart from "up to 1 Watts" that may not even give you enough output voltage and highly likely the speaker negative output terminal is connected to common ground as it is only designed to drive transducers.Instead you could have a look at the commercial audio manufacturers that make unbalanced to balanced audio converters such as:https://www.sonifex.co.uk/redbox/rbul1_ld.shtml24dBu output is fairly common, 28dBu or anything above not so much.

author: pope
author_rank: Frequent Contributor
timestamp: November 16, 2024, 10:37:49 am
content: Quote from: The Soulman on November 15, 2024, 11:22:58 pmThanks for that. I remember coming across this device many years ago.The RB UL2 is a bit tempting as it's reasonably priced but I think the specs are not up to the task.The +28dBu output is really just a ballpark figure. At this point, anything >= +25dBu should suffice, I suppose. I will only use it a handful of times per year.