Poor Man's Pultec EQP1-A Build Support Thread

[MatthisD]

Is there enough space to drill our own mounting holes at the corners? not at the inductor one I know.

Regarding the Q max resistor, is it selected for the particular inductance/capacitor combination? and is the range of the sharp/broad setting reduced as a result?

Are you using the 1H tap of the inductor for the 3K setting and a 2.8nF capacitor(s). I had looked at using the poorman in original Pultec form a while back but thought for the 3K setting I could use 600mH/4.7n combination with a 10k Q pot.
Thanks.

Here is a pic of the single PCB Pultec 3 band EQ. The hi boost/cut switch is front left with it capacitors behind i and the (optional) inductor behind it. The mid bboost/cut switch is centre front with it capacitors behind it and the inductor behind them. On the righ front is the lo boost/cut switch with its capacitors behind it and behind that is the pad you need for the EQ in/out switching.

There are no pots on the PCB just the switches which are on exactly the same centres as the Helios EQ version of this PCB. All the pot connections are wired to a triangular shaped trim pot pad footprint in an attempt to make it fairly obvious how to wire each one up to its associated pot.

Cheers

Ian

 

[ruffrecords]

Is there enough space to drill our own mounting holes at the corners? not at the inductor one I know.

There should be. If you are not using the PCB mounted switches then you should be able to place mounting holes near SHi and Slo. There's also room for one between the two inductors and one near the pad circuit. I nearly put mounting holes in on the PCB layout but I thought that if I did then someone would want them somewhere else.

Regarding the Q max resistor, is it selected for the particular inductance/capacitor combination? and is the range of the sharp/broad setting reduced as a result?

It depends really on the inductor as its resistance sets the maximum Q. In the original Pultec this resistor did not exist and the maximum Q turns out to be around 3. I put this in to allow for inductors with lower series resistance that might need padding out to get the Q closer to the original. You can also use this in the poor man's version of the hi boost as the resistor in series with the wiper of the pot.

Are you using the 1H tap of the inductor for the 3K setting and a 2.8nF capacitor(s). I had looked at using the poorman in original Pultec form a while back but thought for the 3K setting I could use 600mH/4.7n combination with a 10k Q pot.
Thanks.

The 1H tap (F) on the inductor is not used at all so the 3K setting would be as you propose.

Cheers

Ian

 

[MatthisD]

It depends really on the inductor as its resistance sets the maximum Q. In the original Pultec this resistor did not exist and the maximum Q turns out to be around 3. I put this in to allow for inductors with lower series resistance that might need padding out to get the Q closer to the original. You can also use this in the poor man's version of the hi boost as the resistor in series with the wiper of the pot.



The 1H tap (F) on the inductor is not used at all so the 3K setting would be as you propose.

Cheers

Ian

Ok, and would the inductor resistance need to be proportionately higher, as are the pot values compared to the original or is the Q independent of circuit impedance? I'm going to wind the inductor so I can work out the resistance from the core AL value/wire gauge combination but I'm just a bit unsure of the impedance/Q relationship.
It'd be great to get a few of the PCBs if and when you have them available.

 

[MatthisD]

I'm just a bit unsure of the impedance/Q relationship.

I think I understand it a bit better now, with a resistance of 370ohms (including inductor resistance) the max Q is about 3 for 600mH/4.7n. I'm not sure if thats right, I can try it with a 150mH inductor for the 12Khz and see what it looks like.

 

[ruffrecords]

I'm just a bit unsure of the impedance/Q relationship.

I think I understand it a bit better now, with a resistance of 370ohms (including inductor resistance) the max Q is about 3 for 600mH/4.7n. I'm not sure if thats right, I can try it with a 150mH inductor for the 12Khz and see what it looks like.

The Q of the circuit depends on two factors. The first is the total resistance across the LC resonant circuit. This includes the resistance of the inductor itself but also resistances in the rest of the EQ circuit and the source and load resistances. It is not at all easy to work out what this total resistance is and it will vary as the other controls are altered and with different source and load resistances. The designers job is to try to make the overall circuit resistance as constant as possible and to be affected only a little by other controls and source/load resistances.

The other factor is the characteristic impedance of the LC circuit which is the impedance represented by the L and the C at resonance and for the inductor is equal to Xr = 2*pi*f*L where f is the resonant frequency and L the inductance. If the total resistance across the LC circuit is Rt then the Q is just Xr/Rt.

To keep the Q constant you therefore need to try to keep the characteristic impedance constant which really means a separate LC combination for each frequency. This is rarely practicable and so most passive EQs compromise by using one inductor for several frequencies. The Helios LC bass boost does this with a single value inductor and a switched capacitor. The Q gets higher as the frequency gets higher but the bandwidth at each setting is constant. For this reason this is sometime called a constant bandwidth EQ.

Hope that helps.

Cheers

Ian

 

[MatthisD]

Thanks for explaining, with a separate LC for each frequency you can maintain the Q and if doing this the value of the Qmax resistor becomes a more simple choice.

With higher impedance in your version means that Xr has to be raised for Xr/Rt to be similar to the original. If you raise Xr by lowering the capacitor/raising L then the cap value for the high frequencies gets very small, would that be why you've used a 10k pot instead of any higher?
Sorry to drag it out!

 

[ruffrecords]

With higher impedance in your version means that Xr has to be raised for Xr/Rt to be similar to the original. If you raise Xr by lowering the capacitor/raising L then the cap value for the high frequencies gets very small,

Correct.

would that be why you've used a 10k pot instead of any higher?
Sorry to drag it out!

If you compare my circuit values with the original Pultec EQP1A ones you will see that just about everything has been changed by a factor of  about 4.7. The inductors are about 4.7 times bigger and the capacitors about 4.7 times smaller. In the same way, the treble boost pot has been increased 4.7 times from 10K to 47K. The original Pultec Q pot was 2.5K which times 4.7 is actually 11.75K so 10K is the nearest most common value pot to use.

In the original Pultec there is no Qmax resistor so the maximum Q is determined by the circuit resistance and the inductor resistance.

Now inductance is proportional to turns squared so if you double the turns the inductance goes up by four times. But resistance is proportional to the number of turns so if you double the turns you only double the resistance which would therefore lead to a higher Q. However, to get more turns on you often use thinner wire which has a higher resistance which lowers the Q. The bottom line is we have no way of knowing if the resistance of present day inductors using my design has scaled such that the Q remains the same as the original. Given modern materials are likely to provide more inductance per turn then it is likely that they have a higher Q. So the  Q max resistor is included to compensate for this improvement and to try to limit the maximum Q to the same as the original.

Having said all that, it is highly likely that the maximum Q is determined largely by the circuit resistances external to the inductor so I expect the Q max resistor could in most cases be set to zero and no one would notice the difference. It is in there just in case.

The Q pot achieves maximum Q at minimum resistance and minimum Q at its maximum resistance.

Cheers

Ian

 

[surfkat]

I would like to build a few of these with unbalanced in and out so I can hang them off of the unbalanced channel inserts on my Soundcraft Delta console. I am thinking of using a jfet input opamp for a gain stage (TL071, OPA134). I understand I need about 24dB of gain.

Do I need to have a buffer amp for the input or can I plug the insert send directly into the passive EQ? I could easily use a dual opamp and have 1/2 for a input buffer if needed.

Will I need a coupling capacitor after the gain stage? Looking at the schematic for my board, the first component in line after the insert return is a 47uF electrolytic.

Thanks for any thoughts and/or suggestions.

Special thanks to Ian for all the work he has put into this project.

 

[ruffrecords]

I would like to build a few of these with unbalanced in and out so I can hang them off of the unbalanced channel inserts on my Soundcraft Delta console. I am thinking of using a jfet input opamp for a gain stage (TL071, OPA134). I understand I need about 24dB of gain.

That's about right.

Do I need to have a buffer amp for the input or can I plug the insert send directly into the passive EQ? I could easily use a dual opamp and have 1/2 for a input buffer if needed.

Soundcraft insert points generally have a nice low send source impedance so I would say you should be able to drive the EQ directly.

Will I need a coupling capacitor after the gain stage? Looking at the schematic for my board, the first component in line after the insert return is a 47uF electrolytic.

You almost certainly do not need one but there is always the possibility your circuit develops a fault which might possibly damage the Soundcraft. So to be safe I would include an output electrolytic capacitor followed by a resistor to ground (10K would do)

Cheers

Ian

 

[MatthisD]

I've been using the EQ with some different options for gain makeup that each seem to work well.
EQ output - Jensen DI transformer - API preamp
EQ output - Bo Hansen DI with Jensen as above - API preamp
EQ output - API(3124+) Hi-Z input which is specified as 470K impedance.

The first two I couldn't tell apart tonally with the gain also equal between the two. With the DI buffer load (1M) on the EQ I measured the low boost 47K pot to be 25k total Resistance, with just the DI transformer load(200k approx) it measured 17.3K total. I'm wondering if theres something wrong or if thats just the load in parallel with the circuit impedance in parallel with the low boost pot.

The third way was tested at a friend's studio and I didn't have a meter with me. He ran it through Waves 'Q-clone' plugin which displayed the response curves and they were as expected. At the moment its on a wooden panel, a 1/4 inch output jack and there is some noise but so far it sounds great with either of the above for gain makeup, I've yet to try it with the PM's tube gain makeup.

 

[ruffrecords]

The first two I couldn't tell apart tonally with the gain also equal between the two. With the DI buffer load (1M) on the EQ I measured the low boost 47K pot to be 25k total Resistance, with just the DI transformer load(200k approx) it measured 17.3K total. I'm wondering if theres something wrong or if thats just the load in parallel with the circuit impedance in parallel with the low boost pot.

That's probably about right. If you have a transformer at the input its secondary dc resistance is quite low so the input end of the high boost pot is effectively connected to ground at dc So you have the 47K of the hi boost pot in series with the 4K7 of the hi cut pot, in parallel with the 47K of the lo boost pot which is about 25K.

Cheers

Ian

 

[dirtyhanfri]

Some Work In Progress...

I've wired one channel, hope everything is fine....

The PSU (More about it below...)

The (nice) tubes...

Recicled box from a dead shitty hifi-amp...

Now, the problems....

I've fired up the psu, with nothing connected to it, (Also I forget the "A" Connection to chassis as I noticed later...) and while I was trying to measure V at the outputs, the transformer started getting hot, smelling too bad and smoke going out from it...., after it, I turned it off, unsoldered the psu from the transformer, fired it up again and no no voltages coming out of it.... No fuses were used at that moment... I know...

Could it be because I didn't connect the 0V of the psu to the chassis?

The transformer was correctly connected to mains power...

Any ideas about that?
Thanks

 

[ruffrecords]

Sorry to hear about your smoking transformer. This should not be caused by not connecting the 0V of the PSU to the chassis so don't worry about that. It sounds like one of the secondaries was shorted somehow.

Many mains transformers have a built in thermal fuse as a safety measure to stop them causing a fire. Unfortunately it is like a fuse (it cannot be reset) and it is usually buried deep inside the transformer. Disconnect the transformer from everything and check the primary winding resistance. If it is open circuit then you have blown the thermal fuse I am afraid and the transformer is scrap.

Apart from that your build is looking good!

Cheers

Ian

 

[dirtyhanfri]

Apart from that your build is looking good!

Thanks!, I'm trying to give it the look it deserves... (almost one year working, ocasionally on this)

About the transformer, I'm a bit worried about the PSU home made pcb, maybe the 250v solder leads are so close?, disconnected from everything, my multimeter doesn't read continuity between them.... (I put electrician tape trying to prevent contact between traces and chassis, but I don't know if it does anything more than look like crap....)

Does the transformer atachent system need to be isolated from the chassis?

I will try to measure R between main leads, but I'm not so hopefull, I could hear some melting noise apart the smell, but one can never know...

Also I will check for continuity and other issues in the psu, thanks for the tips...

 

[ruffrecords]

About the transformer, I'm a bit worried about the PSU home made pcb, maybe the 250v solder leads are so close?, disconnected from everything, my multimeter doesn't read continuity between them.... (I put electrician tape trying to prevent contact between traces and chassis, but I don't know if it does anything more than look like crap....)

For HT that is probably not enough.I prefer to mont the HT PCB on insulated (nylon) pillars

Does the transformer atachent system need to be isolated from the chassis?

No, it does not need to be isolated from the chassis. However, if the chassis is steel (rather than aluminium) the transformer will sometimes hum if bolted direct to the chassis and sometimes magnetic interference can be transmitted from the mains transformer via the steel chassis to the input transformer which we do not want. For steel chassis I normally put a rubber grommet between the transformer and the chassis using its mounting bolts.

I will try to measure R between main leads, but I'm not so hopefull, I could hear some melting noise apart the smell, but one can never know...

Also I will check for continuity and other issues in the psu, thanks for the tips...

Good Luck.

Cheers

Ian

 

[dirtyhanfri]

Finally, the transformer was dead  :-\

I've finished the wiring of everything, so just waiting the transformer to start trying voltages and such...

Any tips on trying the psu for not burning another transformer? I replaced the 4 diodes for a little bridge rectifier (400V), and I have one question, the pin labelled "+" goes to the 1Watt resistors and + pin of caps, right? And the pin labelled "-" goes to - side of the caps?

I will upload a photo showing the copper side of my home made pcb later and the wiring scheme I'm assuming...

Thanks

Edit: thanks for the correction Ian, I use to confuse both names...

 

[ruffrecords]

Finally, the transformer was dead  :-\

I've finished the wiring of everything, so just waiting the transformer to start trying voltages and such...

Any tips on trying the psu for not burning another transformer? I replaced the 4 diodes for a little regulator (400V), and I have one question, the pin labelled "+" goes to the 1Watt resistors and + pin of caps, right? And the pin labelled "-" goes to - side of the caps?

Yes, that is correct and I think you mean a bridge rectifier rather than a regulator.

The best tip I can give you is to try the HT supply on its own first before connecting it to the tubes. Just use a 300K 1 watt resistor across the output as a temporary load. Turn the power supply and check the output voltage across the 300K load is over 300 volts. Turn it off and leave it for at least 30 minutes to discharge the capacitors, then connect up to the tube gain make up stage.

Another tip is that sometime the inrush current into the first electrolytic capacitor causes the first 1K 1 watt resistor to get very hot and even sometime it will smoke. To be safe you should use a 2W or even  a 5W type here.

I look forward to more pics.

Cheers

Ian

 

[dirtyhanfri]

Thanks for the tips

It is burning fuses, Is 500mA enough, right? I think I readed it somewhere here, but I'm not sure right now, even out of the metal case, over my wood board it gets shorted, but I can't see any electrical damage in any of the comps or the board, so I'm not sure

I realized my home etch pcb is so crappy, so I will not use it anymore, I will dessign a pcb using eagle (I was looking for time to learn using it, this looks like a nice chance) and will upload here when I got it, I'd like to make a nice job with this eq...

Thanks again for the tips

 

[wave]

I think I had spec'ed a 250mA fuse for mine. I haven't tried it yet tho.
Can you post a photo of the etched side of your PSU board? Have you triple checked for any shorts that may be happening on the PSU board?

I have an etch file that I'm going to use and you can try it if you want to

Dave

 

[dirtyhanfri]

I think I had spec'ed a 250mA fuse for mine. I haven't tried it yet tho.
Can you post a photo of the etched side of your PSU board? Have you triple checked for any shorts that may be happening on the PSU board?

I have an etch file that I'm going to use and you can try it if you want to

Dave

I could upload a picture but it's so ugly, and I made a few track cutting for adapt the rectifier, so it's uglier now...

Of course, i have checked everything (many times indeed) looking for shorts or something, but couldn't find anything.

That etch file would be great, thanks, this night I've been struggling with eagle, I almost got it, surely I will finish it for the learning process...

One question, the first capacitor, before the rectifier, should it be a electrolityc, right? the one I have is a small electrolitic, but labelled 100uF 250V, it seems strange to me, it much smaller than the other caps in the psu...

Thanks!