Portable two-band Parametric EQ (ESP/Urei 545)

[domingo]

I'm sorry for asking too much. Since I'm going to be exposed to cellphone signals often (600-4000MHz) it worries me particularly in that range, but your comment already gives me a start. I'm not using XLR connections here, but I'll deep into it.
At the moment I'm very satisfied because I managed to set Q as I wanted, thanks to your advice. I ended up removing R6 (wired) and reduced R11 to 330ohm; no distortions, oscillations or noise and still a very wide range possible. I would get you some beers if possible! Deep deep gratitude.

 

[neil.johnson]

If you're running off batteries you have (at least) two issues:

• Low headroom
• Power consumption

For the first you want to be looking for opamps that can operate near the rails. So far all the opamps mentioned in this thread can only drive within a couple of volts of each rail. With the three batteries you have about 10 V, so losing 4 V limits your headroom to 6 Vpp, about +2.7 dBu. And over time as the battery voltage drops you lose even more headroom.

The second factor directly affects how long your batteries will last. You want to try and minimise the quiescent current. As this is a filter circuit rather than plain gain, I would look at the TLV9102. As a fall back look at the TLV9152.

And your concern about RFI - start with a shielded case and decent connectors with a solid connection to the case, and ideally some sort of filtering in the connectors themselves.

Neil

 

[domingo]

Thank you for your interest Neil. Funny that you mention it because I've just left the filter ON overnight by accident and now I'm having to recharge all batteries. Strangely enough one in the series was much more discharged than the others, I wonder why...

It is true what you say and I'll consider looking for low quiescent current opamp alternatives. Unfortunately the models you mention are not easy to find here. But I'm curious about how sound quality might be affected by lower power/general purpose opamps, because sound quality is my main concern as batteries can always be larger or I could add another one to the series to increase headroom

As for RFI... should I always keep my ground (0v) plugged to the shielded case? Doesn't this make it also more vulnerable to pick up noises from outside? I had a bad experience once with a mic preamp attached to a camera being very noisy until I detached ground from its case.

Thanks in advance!

 

[abbey road d enfer]

If you're running off batteries you have (at least) two issues:

• Low headroom
• Power consumption

For the first you want to be looking for opamps that can operate near the rails. So far all the opamps mentioned in this thread can only drive within a couple of volts of each rail. With the three batteries you have about 10 V, so losing 4 V limits your headroom to 6 Vpp, about +2.7 dBu. And over time as the battery voltage drops you lose even more headroom.

The second factor directly affects how long your batteries will last. You want to try and minimise the quiescent current. As this is a filter circuit rather than plain gain, I would look at the TLV9102.

I agree with all you said, but 28 nV/√ Hz at 10 kHz may be a stumbling block. Actually, really low noise design often disagrees with low power consumption.
Since I don't know exactly the goal of the OP, I may be very wrong.

 

[neil.johnson]

I agree with all you said, but 28 nV/√ Hz at 10 kHz may be a stumbling block. Actually, really low noise design often disagrees with low power consumption.
Since I don't know exactly the goal of the OP, I may be very wrong.

Yes we need to know the spec before going much deeper. As this is an EQ that helps the bandwidth management. But if lower noise is needed then perhaps the TVL9152 which is 10.5 nV/√ Hz although it's a bit toastier at 560 uA per channel.

Neil

 

[domingo]

What are the consequences of reducing the 22K resistors in the ESP diagram to 2K? That means R1, R2, R4, R5, R7 and R8 (I attach the original diagram here again).

I've tried this to reduce potential noise and the filter seems to respond easier now to pot changes. Have I also reduced overall impedance as well? Frequency response seems unaffected.

Thanks again for backing up my lack of knowledge.

 

[abbey road d enfer]

What are the consequences of reducing the 22K resistors in the ESP diagram to 2K? That means R1, R2,

Doesn't change much, except you're increasing the load, which may be an issue with low power opamps and battery drain

R4, R5, R7 and R8

It increaseses significantly the BandWidth, about 5 times. The BW pot cannot compensate that.

Frequency response seems unaffected.

I find it hard to believe.

 

[domingo]

You are right and that is quite embarrassing. I forgot checking Q and it was above the clouds.

Roll back. Would you still recommend Eliott's 22K values if you were to cascade two filters? I wonder because the original Urei, which cascades many, uses a standard value of 5.1K. Therefore my silly experimentation.

 

[abbey road d enfer]

Although the definition of a parametric EQ is "no interaction between parameters", it turns out that most of the components have large interaction. R6, R11 and VR2 should be decreased in the same ratio.
You have decreased by a factor 10. Actually, unless you plan to use VLN opamps (that draw too much current), no use in going lower than 5k (factor 4).

 

[domingo]

I see. In that case I'll stick to 22k as a base.
Intending to increase gain I lowered R4 from 22k to 2.2k, because I need sometimes extreme curves to detect the frequency of certain sounds. According to my understanding the gain of the filter depends on the relation R5/R4, so in this case I should be getting (and I think to be experiencing it) an increase of gain from "1" to 10 (+20dB). Is that correct?
After increasing gain the BW curve seems to have changed as well. Even with the gain pot close to the centre I don't get the same Q as when R5 equals R4, at similar "volume" or output RMS voltage. Interactions might explain that I guess. As a workaround I'm thinking to leave R4 in 2K and add a switchable 20K in series between it and the pot.

 

[abbey road d enfer]

With R4 at 22k, you get about 10dB B/C. With 2.2k, you would get about 27dB. However, the curves move slightly homothetically, which means you touch a lot of distant frequencies. You would need to increase the "Q" to compensate, but I think the opamps would run out of steam.

 

[domingo]

Thank you for your information Abbey. I expanded BW to the maximum setting possible I found. What ended up working for me was to set R6 at 100ohm, R11 at 330ohm and add a 1K between VR2 and R4/R5 to avoid oscillation. R4 can stay then at 2K without distorting or oscillating by itself in all pot positions, but still might add an optional 20K in series to R4 for more gain resolution when subtle tuning is rather needed.
It's good to know the gain levels I'm having. How come do you calculate it? Does it depend on a sum of all opamps interacting? I would appreciate some info about it to be able to do my own calculations.

I'm also wondering about C4/C5. Those 100u caps were added by ESP as they are not present in the individual filters of the Urei 545. I suppose they are coupling caps to filter possible noise coming from the ground line? :s From a practical point of view, I'm asking myself if it is necessary to have them for each filter. I'm cascading two filters in stereo mode, which makes up to 4 filters. Since they all have a common ground, could they not all share one pair of caps instead of having to use 8? This would reduce my build size considerably.

Have a nice weekend and end of the week!

Domingo

 

[abbey road d enfer]

I'm also wondering about C4/C5. Those 100u caps were added by ESP as they are not present in the individual filters of the Urei 545.

I don't know. These caps are not needed. They may be for a single-rail design, but there would be other caps.

I suppose they are coupling caps to filter possible noise coming from the ground line?

No. These points (bottom of R12 & R13) would go to ground with dual rail power, but they should go to the half-voltage rail in a single rail. This half-voltage rail must be stiff, so I would recommend by-passing it withat least 1000uF.

 

[domingo]

Thanks. Since I'm using dual rail now I can safely remove them then. I experienced no difference in sound without them actually.

(I modified the title of the thread so it's easier to find for people looking for similar information)

 

[JohnRoberts]

Back in the 1970s I sold a parametric EQ kit (Phoenix Systems P-94). The basic kit was provided with +/- 15V supply but I also included notes in the Popular Electronics kit article about operating the P-94 from single 12V supply, for automotive use.

I obviously no longer sell or support these kits but the original Popular Electronics kit article may be found floating around the internets. IIRC it was published in the fall (Sept?) of 1979.

JR

 

[nille]

https://www.scribd.com/document/25859424/John-Roberts-Parametric-EQ-Audio-Project

 

[carlmart]

What values would be best for a low frequency parametric boost?

I believe there a famous classic loudspeaker that included, at leas as option, such a parametric to add before the power amp. It's still an interesting idea for smaller box loudspeakers. Much better than a simpler bass equalizer.

 

[abbey road d enfer]

What values would be best for a low frequency parametric boost?

I believe there a famous classic loudspeaker that included, at leas as option, such a parametric to add before the power amp. It's still an interesting idea for smaller box loudspeakers. Much better than a simpler bass equalizer.

Your question is quite vague. You may find relevant info there:
https://en.wikipedia.org/wiki/State_variable_filterhttps://sound-au.com/articles/state-variable.htm

 

[Ricardus]

I don't want the EQ, but I'm all about that TimeGlow digital alarm clock!

 

[domingo]

Here is the schematic of the 2-band (full range) parametric EQ I've been working on, with the collaboration of everyone in this thread. Took me some time to find the resistor values to make it full range and had to put the build on hold for some time, since it takes part on a larger project.

Only one channel is showed of course.

Power supply are 3x 3.7V Li-Ion in series. At min. voltage (9V) the rail splitter sits at 5.5V, still allowing a 3V swing (4V-9V) within the common mode voltage limits of an AD712/TL072.

A mixer to fully bypass the EQ was added at the end. I partially tested it and it worked ok, but I'm not sure if decoupling caps should be added between the opamps below (U30, U25, U45). I also have to try with the two filters cascaded (only tried with one).

It is still not clear to me if all grounds marked as VGND are correct, since I think it was mentioned in this thread that at least the ones after the 100u caps should go to normal GND (-6V). Maybe I misinterpreted it?

Any contribution or comments will be highly appreciated. My idea is to document here any improvements.

Thanks a lot for the time and knowledge given.