UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Reply
 
Thread Tools
Old 6th Sep 2020, 10:49 pm   #1
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,086
Default Op-Amp RIAA Preamp with Tone Controls - Help!

I followed this thread with interest https://www.vintage-radio.net/forum/...=169345&page=2 since I wanted to build an improved op-amp RIAA pre-amp with tone controls. I opted for the one that appeared in Post #40 of the above thread and is reattached below (Douglas Self design I believe). I approximated some of the component values for ease of construction and redrew the circuit as shown in the second attachment. I used an OPA4134 quad op-amp and a split rail supply +16v/-16v. Well, I'm getting sound out of it but the top end frequencies are being severely attenuated. Even with the treble at max lift, the treble response is very poor whereas the bass is booming. I put the sig gen and scope onto it. For a 15mv pk-pk signal input at 100Hz I'm getting about 1v pk-pk at the output whereas at 200Hz it is down to 300mv - the roll-off is very rapid. At higher frequencies the output diminishes still further. The second part of the circuit (the one with tone controls) seems to be working correctly, because transferring the scope to the output of the first stage (RIAA pre-amp) reveals the rapid roll-off is occurring there. I have built the circuit on veroboard and as far as I can see I have wired the circuit correctly. The problem occurs identically on both left and right channels. I've probably made a schoolboy error somewhere. Can anybody point me in the right direction? One thing that bothers me is that the feedback capacitors seem extremely high compared to other RIAA circuits that I have seen. Cheers, Jerry
Attached Thumbnails
Click image for larger version

Name:	RIAA Op Amp.jpg
Views:	132
Size:	47.8 KB
ID:	215098  
Attached Files
File Type: pdf RIAA Pre-amp+active filter.pdf (82.1 KB, 47 views)
cathoderay57 is offline   Reply With Quote
Old 6th Sep 2020, 11:50 pm   #2
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 15,634
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

The capacitor values along with the resistor values must implement what some people call the specified time-constants of the RIAA characteristic. Other people talk of the pole and zero frequencies. Same thing, anyway.

So big capacitor values will have to go with scaled-down resistor values.

Resistors produce a set available noise power at a given temperature. How this relates to voltage comes from V squared over R. So higher value resistors give more noise volts.

This is mostly all there is to worry about. The use of a low R/high C network is better for noise contributions from the resistors.

The NE5534 is a very good opamp which well suits the source impedance that a cartridge presents to it. The low R type of RIAA variants carry this through to making the feedback (series and shunt) elements present a similar impedance.

The Shure brothers ask you to load their cartridge with 40k or whatever. You need to create this load with a resistor to correctly damp the resonances of the cartridge. This does not mean that the cartridge will look like 40k to the amplifier. On the surface of it, it looks like there is a big mismatch going on, but there really is a reasonable noise-match.

The conditions for optimum gain and optimum noise figure rarely coincide. Life would be too easy...

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 7th Sep 2020, 9:56 am   #3
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,086
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

Hi David, thanks for the reply, and helpful explanation of the negative feedback implementation. The cartridge I am using is a Chuo Denki YM-118, also listed as CEC MC-11 and Harksound 8001. I can still get replacement stylii but there is no data anywhere on this cartridge (it came fitted to my Thorens TD150 Mk II turntable). I have seen it described as "piezo" but that is wrong because it is definitely magnetic having dc resistances of 387R and 394R. I suspect it is a moving magnet type. However, without any data, I cannot find exactly what impedance I should be matching it to. The old version of my preamp is attached and the performance using the same cartridge was excellent with plenty of bass and treble lift (I know, if it ain't broke don't fix it). I guess if push comes to shove I can update the old circuit to split-rail which would perhaps improve noise performance to some degree by removing the offset resistors. With the new circuit I'm getting enough gain to drive the PA which is a valve unit with push-pull EL34s, ECC83 phase splitter and triode-connected EF86 pre-amp. Any further advice much appreciated. Cheers, Jerry
Attached Files
File Type: pdf RIAA Pre-Amp (Old).pdf (83.2 KB, 39 views)
cathoderay57 is offline   Reply With Quote
Old 7th Sep 2020, 10:46 am   #4
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 15,634
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

The TL07x opamps are an absolutely lousy choice for a magnetic cartridge. Their FET inputs are optimum for noise with very large source impedances. They would be OK for use with piezo-electric cartridges.

One comment about split-rail supplies in the TL074 circuit you posted is that the DC output voltage from IC1 is very close to ground, and due to offset voltage in the opamp. It could be a little bit positive, or a little bit negative. This is not good for either C5 or C4 and the equivalents in the other channel.

Aluminium electrolytics degrade (depolarise) if they have no bias. Tantalum electrolytics will survive, but they have their own problems.

With the NE5534 circuit, the offset voltage gets an added component of the bias current times the DC impedances of the circuits attached to the + and - inputs. Say 63k Ohms times 1uA so 63mV. Still not much, but watch out for bias on electrolytic capacitors if you want reliability.

I don't see anything in the RIAA circuit that should necessarily limit the range of your tone controls. I'd definitely use the NE5534 RIAA circuit and then treat tone controlling as a second issue.

The baxandall circuit assumes that the source impedance driving it is fairly low, so the high impedance (100k) volume control will cause the tone control settings and characteristics to wander as you change volume.

The NE5534 has no problems with driving things, so a 10k pow for volume could be somewhat better.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 7th Sep 2020, 1:02 pm   #5
Ted Kendall
Nonode
 
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
Posts: 2,330
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

The RIAA curve drops about 20dB between 50 and 500Hz, so about 8dB beween 100 and 300Hz isn't wildly out. Are you absolutely sure you have a moving magnet cartridge?
Ted Kendall is offline   Reply With Quote
Old 7th Sep 2020, 1:12 pm   #6
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,086
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

Quote:
Originally Posted by Ted Kendall View Post
Are you absolutely sure you have a moving magnet cartridge?
No, but short of dismantling it and in the absence of data, hard to tell. If it has dc resistance then surely it must contain coils, not crystals? David, note that in my new circuit, albeit based on that for a NE5534, I am actually using an OPA4134. See below. Cheers, Jerry
Attached Files
File Type: pdf opa4134.pdf (1.31 MB, 23 views)

Last edited by cathoderay57; 7th Sep 2020 at 1:23 pm.
cathoderay57 is offline   Reply With Quote
Old 7th Sep 2020, 2:53 pm   #7
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,086
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

OK, here is the measured gain at the final output resistor (68R) of the Self circuit, using one chip of the 4 on the OPA4134, and a 15mv pk-pk input from a sig gen with 600R output:
100Hz 43.7dB; 200Hz 38.8dB; 300Hz 36.5dB; 400Hz 34.5dB; 500Hz 33.4dB; 600Hz 32.0dB; 700Hz 32.0dB; 800Hz 31.3dB; 900Hz 30dB; 1kHz 30dB.
I'm guessing that is not too bad. The one thing I've noticed is at the front end I have got a 68k resistor across the non-inverting input and a 150k across the cartridge. Received wisdom seems to be these 2 components should be transposed so I will try that and see if the change in cartridge loading makes any audible difference at the top end.
cathoderay57 is offline   Reply With Quote
Old 7th Sep 2020, 3:45 pm   #8
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 15,634
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

You're hacking randomly!

Stop and think. If the decoupling/DC block capacitor is big enough, the cartridge sees both resistors in parallel and cares not one jot which order they come in.

The opamp, if it has any bias current on its inputs will drop some voltage in the resistor its side of the DC block, so you put the lower resistor here. The resistor on the cartridge side of the DC block just acts as a charge path for the DC block to stop pops and bangs when you plug things in.

David.
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 7th Sep 2020, 3:48 pm   #9
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,086
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

I'll get my coat....
cathoderay57 is offline   Reply With Quote
Old 7th Sep 2020, 4:49 pm   #10
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 3,886
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

While I haven't read every single post in the thread so far, there is one thing that leaps out at me: you've put the tone controls immediately after the phono pre. That won't work - you need a buffer between the two stages.

The reason for that is the input impedance of the tone control stage varies with frequency. It also varies with the position of both tone controls. More on that in the Small Signal Design book from Self.

Meanwhile, the output impedance of the phono stage is set by R8 and R9 - these two are working with C5 to form a HF pole, of course - and the volume control. I'm not sure if the value of this is specified, but at mid position, the source impedance it presents will be quarter the value of the track if the top is fed from a low impedance source - in this case it'll be greater because of R8 and R9.

So temporarily disconnect the tone control stage (lift one side of C6), and take an output from the wiper of the volume control. See how that sounds working into a known-good hi-fi setup that sounds OK with a CD player or similar. That should prove that the RIAA stuff is OK (I haven't looked in detail at your component values there yet). Then, add a buffer between the volume wiper and C6. With any luck that'll do the job

I remember finding out about this the hard way! That was before I was comfortable with op-amps, so I fixed it with a simple emitter follower. Happy days
mhennessy is offline   Reply With Quote
Old 7th Sep 2020, 6:00 pm   #11
Craig Sawyers
Nonode
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 2,854
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

The 1960's Quad 33 had 6 transistors per channel. Two in the RIAA, one emitter follower with taps on the emitter resistor to select output for tape recorder. That is followed by the volume control (50k), and then a bootstrapped input (so it doesn't load the volume control) emitter follower. The low impedance output from that feeds the Baxandall tone control wrapped around a bootstrapped transistor pair.

They packed a lot of subtlety into 6 transistors per channel.

But the use of an emitter follower feeding the volume control, and a high impedance bootstrapped emitter follower after it to feed the Baxandall circuit is exactly the right way to do it.

Just substitute unity gain op-amps before and after the volume control and that should sort the problems - at least in that area!

Craig
Craig Sawyers is offline   Reply With Quote
Old 7th Sep 2020, 8:36 pm   #12
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,086
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

Thanks all for the suggestions and guidance. Sounds like version 3 of the unit is needed. I'll let you know where this gets to. BTW, I did fit a (hopefully) better quality cartridge to the turntable, an Audio Technica AT100E that I bought some time ago but never used. At least the cartridge is now a known quantity. Cheers, Jerry
cathoderay57 is offline   Reply With Quote
Old 24th Sep 2020, 9:32 pm   #13
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,086
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

Hello folks, I have had another stab at building a better RIAA preamp with tone controls. The devices used this time are 2 x NE5534, one for each channel of the RIAA preamp. This is followed by an OPA4134, 2 of the op-amps on it used for unity gain voltage followers (one for each channel) and the other 2 op-amps for Baxandall tone controls. Powered by a plus and minus 15V dc split rail supply. The circuit (page1), veroboard layout (page 2) and an image are attached. Well, it actually works well at low volume, and the original reason for posting here, i.e. poor treble response, has been solved - the treble and bass lift and cut work as they should and the outputs are balanced in volume. Unfortunately, there's the inevitable "but" coming.... If I stop playing music and turn up the volume, the left-hand channel has a very intrusive 50Hz hum. I know it is 50Hz (not 100Hz) because I have put the scope on it. With the turntable cartridge still plugged into the pre-amp, the hum outputs of the two NE5534 chips measured at the junction of R8 and R9 are 10mV pk-pk on the good channel but 140mV pk-pk on the bad channel. Swapping around the turntable leads has no effect. Swapping the NE5534 chips around has no effect - hum stays on channel not on chip. Cross-connecting the NE5534 outputs at the junction of R9 and C16 and turning the PA back on transfers the hum to the other speaker and therefore there is absolutely no doubt that the hum is being generated by the NE5534 shown in the pic at the bottom left; grounding the junction of R8 and R9 for that channel silences the hum. the top left (rear) device in the pic is working as it should, as are both channels of the OPA4134. I tried putting additional decoupling onto the dc supplies to the NE5534s using a 10R and 100uF electrolytic on the + and - rails to no effect. I even disconnected the front NE5534 circuit and replicated the layout of the rear circuit on a daughter board and connected it to the main board as a replacement for the front RIAA channel. The result was the same - the RIAA pre-amp on the daughter board now hums while the rear one still doesn't. Looking again at the image, the use of coax to one of the input sockets and an unshielded wire to other isn't the answer either - the unshielded wire provides the quiet channel! Any thoughts and recommendations welcomed. Cheers, Jerry
Attached Thumbnails
Click image for larger version

Name:	P1080224.jpg
Views:	74
Size:	122.8 KB
ID:	216358  
Attached Files
File Type: pdf RIAA pre-amp with filters.pdf (371.8 KB, 28 views)
cathoderay57 is offline   Reply With Quote
Old 24th Sep 2020, 10:21 pm   #14
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 15,634
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

Proximity to the transformer or to the mains wiring?

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 24th Sep 2020, 10:31 pm   #15
Ambientnoise
Heptode
 
Join Date: Feb 2006
Location: Marlborough, Wiltshire, UK.
Posts: 656
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

A few random thoughts, tracing hum can be a pain. The following might give some clues.

If that’s a neon, then remove it as they can induce hum/buzz. I presume the chassis is connected to mains earth and to 0v at one point ? If the hum goes with shorted inputs then it will be coming in at the very front end. Double check for clean breaks in the veroboard tracks, minute whiskers can give all sorts of problems. Try powering from a couple of 9v batteries and see what happens to the hum. Try unbolting the transformer and insulating from the chassis, but link it to earth. Ensure the metal cases of the pots are connected to chassis. Ensure the board fixings are not causing shorts to chassis.
Ambientnoise is online now   Reply With Quote
Old 24th Sep 2020, 10:42 pm   #16
joebog1
Octode
 
joebog1's Avatar
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 1,215
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

Ambientnoise has the answer for your ambient noise powered by the batteries it will be quiet as.

Joe
joebog1 is offline   Reply With Quote
Old 24th Sep 2020, 11:27 pm   #17
Craig Sawyers
Nonode
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 2,854
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

With the current drain of 20-28mA, the 220uF smoothing caps will produce about 1V of ripple.

Even if the amp had a flat response, the PSRR of 100dB for the devices you are using will produce 1mV of hum. But you get 20dB of bass boost from the RIAA, so ~10mV of hum (actually at 100Hz.)

That it is predominantly at 50Hz suggests that it is induction from that little EI core transformer, which will have a big leakage inductance and spray 50Hz hither and yon.

The other thing that goes hither and yon is your +/- and 0V wiring on the board. For starters, connect those signals to a star connection at the 220uF caps.

Next, use a toroidal transformer to reduce radiated 50Hz, and 7815/7915 regulators to give a decent ripple-free power supply.

Craig
Craig Sawyers is offline   Reply With Quote
Old 25th Sep 2020, 12:42 am   #18
Top Cap
Octode
 
Top Cap's Avatar
 
Join Date: Mar 2006
Location: Watford, Hertfordshire, UK.
Posts: 1,227
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

Here's a design I put together many moons ago, despite all of the hard work I still preferred the all valve version .
One of the big secrets is to use something like +/- 18V otherwise the op-amps run out of puff at the bottom end where lots of gain is required.
http://www.g4cnh.com/public/Ideas_be...amp_build..pdf
__________________
Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!
Top Cap is offline   Reply With Quote
Old 25th Sep 2020, 8:20 am   #19
Craig Sawyers
Nonode
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 2,854
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

Really nice write-up, and good constructional philosophy.

The only thing I'd add is an input capacitor. With a maximum input offset for a 5534 of 4mV, that produces a DC current in the cartridge of the same value as the AC current. I don't know whether that has a deleterious effect, but a suitable capacitor in series with the input is not a bad idea.

Craig
Craig Sawyers is offline   Reply With Quote
Old 25th Sep 2020, 6:27 pm   #20
buggies
Hexode
 
buggies's Avatar
 
Join Date: Jan 2009
Location: West Lothian, UK.
Posts: 492
Default Re: Op-Amp RIAA Preamp with Tone Controls - Help!

Disconnect one end of the screened cable (if both ends connected) - hum loop?
__________________
George
buggies is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 11:55 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.