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Old 5th Aug 2020, 9:23 am   #21
Craig Sawyers
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Default Re: Simple Op Amp RIAA preamp 1st lash up

Apart from resistor and capacitor ratios, the third time constant of 318us is the parallel combination of resistors times the sum of capacitors.

Which for resistors of 560k and 47k, and capacitors of 5.6n and 1.5n equals 308us (error of 3.8%).

Which validates a simple change of R6 from 680k to 560k.

All this supposes that there are no errors in capacitor values of course!

Craig
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Old 5th Aug 2020, 12:00 pm   #22
bikerhifinut
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Default Re: Simple Op Amp RIAA preamp 1st lash up

A crossed wire here, my query on dual vs single rail was aimed at the VSPS circuit that Paul Sherwin originally posted the link to.
http://phonoclone.com/diy-pho5.html
I've got a board on the go with this circuit, having mounted the EQ networks so far after spending a bit of time matching the capacitors and 1% MF resistors.
Heres a thing, the 1% quarter watt Metal films seem to deviate from the nominal value in about the same error as the el cheapo 5% Carbon films I Use in non critical stuff. i.e. the 5% jobs are remarkably close to their nominal value which is remarkable I think.
OK so right now I'm looking at a value of capacitor from R2 to ground, assume that I am using 220R here for the full 50dB, rather than 680R for 40dB. if there's no issues at 50 dB then that's what I shall use. 220R suggests a 220uF capacitor, but if I want to roll off at around 15Hz I think 100 or even 47uF might be more appropriate?
I accept a higher gain means I have to use a higher voltage power supply, currently a 15V AA cell battery pack, and I have another one I can use to make a 15 0 15 V dual supply for now.
But I was doodling around with the circuits and I could see no reason why adapting that simple circuit to work off a single rail shouldn't work.
I was looking to bias the op amp up with a pair of 180k resistors, bearing in mind the noise filter on the inputs there.
The coupling capacitor would then have a 100k across the input side to earth across the cartridge output which would give an overall 47.3k load which isn't a country mile off. that assumes I get a combination of resistors on the top leg of the biasing and noise filter that equals 180k.
Or I forget the 100k which isn't really necessary but nice as mark points out if hot swapping headshells. That's not something I can do anyway but who knows in the future., in that case its fudge around to get 47k load across the input/bias resistors as I like symmetry and seems good practice to reduce DC offsets as much as practically possible and easily achieved using E24 resistors.
I also will put something like 200 to 470uF across the supply to ground whichever stratagem I decide on. And should I put a 0.1uf ceramic or other across the supply pins of the op amps? That seems to be omitted on the circuit and I thought a small value cap across the supply pins as close as possible helped prevent instability?
People new to this thread may be wondering why I am trying another simple chip circuit when the first one seems to work so well. I'm curious, this is nothing more than me discovering stuff for myself.
With masses of help from others here.
Andy
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Old 5th Aug 2020, 12:21 pm   #23
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Default Re: Simple Op Amp RIAA preamp 1st lash up

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And should I put a 0.1uf ceramic or other across the supply pins of the op amps?

Andy
Always a good idea I think and does no harm.

Alan
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Old 5th Aug 2020, 1:08 pm   #24
Craig Sawyers
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Default Re: Simple Op Amp RIAA preamp 1st lash up

If you are putting an electrolytic from R2 to ground to make an LF roll-off that is the wrong way to go. The tolerance on electrolytics is very wide, and they can introduce harmonic distortion.

Bung another stage on and build a Sallen and Key second or third order filter around it using film capacitors.

Anyhow, the RIAA values in the phoneclone are absolulely fine. Time constants all work out very close to correct.

Craig
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Old 5th Aug 2020, 4:20 pm   #25
bikerhifinut
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Default Re: Simple Op Amp RIAA preamp 1st lash up

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If you are putting an electrolytic from R2 to ground to make an LF roll-off that is the wrong way to go. The tolerance on electrolytics is very wide, and they can introduce harmonic distortion.

Bung another stage on and build a Sallen and Key second or third order filter around it using film capacitors.

Anyhow, the RIAA values in the phoneclone are absolulely fine. Time constants all work out very close to correct.

Craig
I wasn't thinking in terms of that being the only reason, but that it is a good idea to put a capacitor there according to advice elsewhere. A quick calculation suggested to me that a 47uF off a 220R resistor got a pole at around 15 hz which was sufficiently low not to affect the LF end.
So would you also say a 1uF at the input is the right value or a bit lower at around 330nF would be more suitable to avoid any excessive LF making it into the circuit.
Its all a bit up in the air until I can get somewhere I can buy some cheap batteries, our village store wants a fiver each for a PP3! And I also want a batch of AA cells.

Ah well maybe I am just making life difficult.

A.
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Old 5th Aug 2020, 4:33 pm   #26
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Default Re: Simple Op Amp RIAA preamp 1st lash up

"Its all a bit up in the air until I can get somewhere I can buy some cheap batteries, our village store wants a fiver each for a PP3! And I also want a batch of AA cells."

I buy my AAs by the hundred from CPC (free p&p at the price, too). Their PP3s are much cheaper as well, if you buy in (smallish) quantity.

Mike
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Old 5th Aug 2020, 4:48 pm   #27
bikerhifinut
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Default Re: Simple Op Amp RIAA preamp 1st lash up

So I think I should concede here and accept that trying to convert the VSPS to single rail is a dead end?
Looks like a trip to get some cheap and cheerful PP3s until I get a few positive voltage regs, I am minded to get adjustable ones rather than fixed as one size then fits all.
Andy.
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Old 5th Aug 2020, 11:35 pm   #28
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Default Re: Simple Op Amp RIAA preamp 1st lash up

So the VSPS is built on perf board using a pair of NE5534 rather than a single NE5532.
Took a bit of fault finding at first, until I twigged. apart from an initial cock up with the feedback connections, I discovered I hadn't actually connected pin6 of either chip which is the output. That explained the total silence then.
Running it off a 15 0 15 supply using a couple of 10 x AA holders, I pinched the extra one out of my battery powered sig gen.
First impressions are that its very quiet on the hiss, bearing in mind I configured it for 50dB gain. using 220R for R2. Sound quality on initial listening and without any loading capacitor on the input seems Very good and not too lively so maybe I'll omit the 100pF loading cap with my standard Ortofon 2M.
There is however a bit of what I think is instability when the wicks turned right up, I hear a "scratting sound" it's regular and therefore its not an intermittent or dry joint I think.
I wont be able to get the Scope on it for a week or two due to being dragged away for a while.
I haven't put a small value ceramic capacitor directly across the power in pins of the chips, so that would be a first resort. After that I wouldn't know where to look for this.
I did do something that probably I didn't need to do and that was to put 47uF in series with R2 to ground, and maybe that's the cause but I doubt it. its an easy fix to simply short out the capacitor rather than rive it out of an already crammed board.
I also used a 0.47 uF at the input as a DC blocker, and a pair of 470uF across each power rail to ground which I felt was sensible. Even though its battery powered.
I wondered if knocking the gain back to 40dB would calm things down a bit, but in my set up 50dB gain is pretty much where I want it.
I started out quite neat but by the end it got a bit scrappy underneath the perf board but I don't think I have any strange wire routing. I will say that I absolutely detest soldering onto the tiny close spaced pins of a DIL 8pin IC holder, and the perf board has a habit of peeling off the copper circle round each perf hole which don't help matters.
If I was to look at this as a keeper of a circuit I think I'd get a PCB done for it. Its early days yet for it.
A.
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Old 5th Aug 2020, 11:38 pm   #29
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Default Re: Simple Op Amp RIAA preamp 1st lash up

thanks boulevardier, I shall investigate for a supply of batteries, I find batteries are useful for some of the stuff I play with.

Andy.
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Old 6th Aug 2020, 12:04 am   #30
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Default Re: Simple Op Amp RIAA preamp 1st lash up

The 100nF between rails is all but essential on these chips to avoid instabilities at ultrasonic frequencies, which sometimes show themselves as a mysterious increase in distortion.
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Old 6th Aug 2020, 12:20 am   #31
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Default Re: Simple Op Amp RIAA preamp 1st lash up

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The 100nF between rails is all but essential on these chips to avoid instabilities at ultrasonic frequencies, which sometimes show themselves as a mysterious increase in distortion.
Thanks Ted.
I dug out a couple of 0.1uF poly films and soldered direct across the + and - of each 5534, i.e. across pins 4 and 7.
hasn't made any difference so something else is causing the noise. and now I have lost the LH channel.
I also shorted out the 47uF to ground off the bottom feedback resistor as per the original circuit diagram in case they were the problem.
It's late so I'll put it all to bed and have another look in a week or so when back.
I have a feeling I will be rebuilding this beastie from scratch but with a bit more care around my soldering, as I wonder if the noise is a dry joint and as I have been using lead free solder its hard to see as they all look dull.
A.
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Old 6th Aug 2020, 1:39 pm   #32
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Default Re: Simple Op Amp RIAA preamp 1st lash up

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So I think I should concede here and accept that trying to convert the VSPS to single rail is a dead end?
I'm confused, which schematic are you actually trying to build? The one in post #2 was single rail, why don't you build that
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Old 6th Aug 2020, 3:32 pm   #33
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Default Re: Simple Op Amp RIAA preamp 1st lash up

And why do you think it's impossible to convert the VSPS to single rail operation? It's really just a bog standard op-amp preamp with carefully calculated feedback components to do the RIAA. If you compare it with any single rail design you'll see what's required - basically, just a couple of resistors.

Lots of stuff on single rail operation here: https://tangentsoft.net/elec/vgrounds.html

Of course, there are advantages to using a dual rail arrangement, and it keeps the component count down.

There is a link to the VSPS circuit in #22 for anyone getting lost.
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Old 6th Aug 2020, 5:51 pm   #34
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Default Re: Simple Op Amp RIAA preamp 1st lash up

Single rail versions of circuits have a side effect of putting bias voltages on some of the capacitors they need. This can cause turn-on and off thumps, but can be handy for keeping electrolytics polarised.

Dual-rail circuits can be lower in thump, but can leave you in a quandary of wanting some large-ish capacitors to roll gain off before DC, large enough to want to use electrolytics, but you can't be sure how much residual DC winds up across them, or what polarity.... a puzzlement. Some people solved it with little bead tants and we're now paying the price.

No one way is perfect, all have different mixtures of compromises.

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Old 6th Aug 2020, 9:08 pm   #35
bikerhifinut
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Default Re: Simple Op Amp RIAA preamp 1st lash up

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Quote:
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So I think I should concede here and accept that trying to convert the VSPS to single rail is a dead end?
I'm confused, which schematic are you actually trying to build? The one in post #2 was single rail, why don't you build that
I already knocked one up based on the velleman circuit and that is the base point. I'm just investigating what difference I can see or hear with different takes on the op amp circuit.
The downside to the Velleman circit is its low gain, which means it will need an extra stage to get the output up to a usable level with my kit.
The VSPS circuit which I have posted a link to already in this thread, and Paul has clarified, thanks for that, seems to have an approval level from the pro engineers that have contributed.
Anyway it isn't working properly. It showed promise but its got some instability issues. I wonder if cranking the gain up to the maximum 50dB hasn't been the best idea...…….That's easily solved.
I'm away for a week from early tomorrow so I will have another go on my return, probably just do a fresh start on a new board as I feel I made this one quite messy and perhaps the issue is one of layout.

Points taken David and Paul on the dual vs single rail thing. I knocked the RJB labs VSPS up as per the article but I do know how to work it single rail which might be worth another look.

OK and finally, given that the EQ network on the VSPS circuit seems to be approved of and is easy to replicate with inexpensive standard value parts, can that be transplanted into other feedback EQ circuits? I can see no reason not to consider it.

back in a week to torture you all……….

Andy
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Old 6th Aug 2020, 11:37 pm   #36
Craig Sawyers
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Default Re: Simple Op Amp RIAA preamp 1st lash up

As mentioned previously the values for the Velleman RIAA are wrong.
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Old 7th Aug 2020, 8:53 am   #37
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Default Re: Simple Op Amp RIAA preamp 1st lash up

Single rail configuration
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Old 7th Aug 2020, 9:26 am   #38
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Default Re: Simple Op Amp RIAA preamp 1st lash up

if you're using the 5534 have you added the 22pf compensation capacitor as gain will fall to 1 at high frequencies?
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Old 10th Aug 2020, 5:08 pm   #39
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Default Re: Simple Op Amp RIAA preamp 1st lash up

I think this thread proves that there is no such thing as a "simple" RIAA circuit.
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Old 10th Aug 2020, 9:37 pm   #40
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Default Re: Simple Op Amp RIAA preamp 1st lash up

The circuits are simple.

It's the people around them that are complex!

David
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