MEQ-5 Low boost problem - Solved!

[dacapitan]

Hello,

I recently completed a Stereo MEQ-5 build using two of the SRPP boards and Franks filter boards. The Mid Freq Cut and Hi Boost work fine, but the low boost does not work at all with any frequencies on either channel - this led me to believe that it must be a wiring issue so I have tried reversing the freq switch and pot wiring but this has no change... I have no clue what is wrong, is there anyone who is kind enough to please help me  out here, I would appreciate it so much!

I see someone else had the exact same problem,  but unfortunately there was no solution ever given I have tried to message him to check if he ever reached a solution, but I am still waiting http://groupdiy.com/index.php?topic=29449.0

 

[ruairioflaherty]

Help us to help you, link to a schematic for the filter board.

 

[dacapitan]

Help us to help you, link to a schematic for the filter board.

Hi ruairioflaherty - thanks for your reply. sure no problem.

the schematic has some modifications from the original. As you can see in this thread, http://groupdiy.com/index.php?topic=27624.0 jensenmann built it with the original cap values but the freq were off some what, so the new cap values have been entered... I have the original schematic cap values in (I am waiting for the new parts) but that should not mean the low boost will not work at all? I will attach the PCB layout in the next post

http://groupdiy.com/index.php?topic=27624.0

 

[dacapitan]

Help us to help you, link to a schematic for the filter board.

PCB Layout

 

[ruairioflaherty]

There are smarter people here but here goes...


1) Have you checked that your 1k resistor to ground actually is a 1k and soldered correctly? (if in doubt about color code lift one leg and check).

2) Take a  test lead/wire and carefully short out the input of the pot (from the transformer secondary) to the input of the inductor in the low boost section while the unit is passing audio.  Are you getting a boost at your selected frequency?

 

[dacapitan]

There are smarter people here but here goes...


1) Have you checked that your 1k resistor to ground actually is a 1k and soldered correctly? (if in doubt about color code lift one leg and check).

2) Take a  test lead/wire and carefully short out the input of the pot (from the transformer secondary) to the input of the inductor in the low boost section while the unit is passing audio.  Are you getting a boost at your selected frequency?

Thanks for getting back to me, I'm sure there is a 1k resistor there and I'm  almost certain it's soldered correctly as it's the same problem in both channels - but I will take a look and also try shorting the pot to the inductor - Its 1am here at the moment so I'll give it a go tomorrow and let you know the result, but appreciate the help so far!

 

[gyraf]

Low boost works by frequency-selectively bypassing the 2k5 part of the voltage divider formed by 2K5(pot) and 1K(resistor).

If your inductor DC-resistance is high, this may add to resistance and give lower boosts.

Jakob E.

 

[dacapitan]

There are smarter people here but here goes...


1) Have you checked that your 1k resistor to ground actually is a 1k and soldered correctly? (if in doubt about color code lift one leg and check).

2) Take a  test lead/wire and carefully short out the input of the pot (from the transformer secondary) to the input of the inductor in the low boost section while the unit is passing audio.  Are you getting a boost at your selected frequency?

Low boost works by frequency-selectively bypassing the 2k5 part of the voltage divider formed by 2K5(pot) and 1K(resistor).

If your inductor DC-resistance is high, this may add to resistance and give lower boosts.

Jakob E.

I tried shorting the input of the pot to the selected inductor pin, but it does not make any difference. I did tried the same methodology to the Mid Cut and Hi boost and I am able to get a cut and boost there.

 

[ruairioflaherty]

Ok.  The aim is to isolate the problem (you could still have a faulty or mis-wired pot but it's not your only issue).

If your 1k resistor is good we can now say for reasonably sure that the issue is in your inductor/switch/cap combination.

Jakob makes a good point about DCR, what inductor did you use?

Things to double check:
- Are you sure you are going in and out on the correct pins of the inductor (pinout may not match your PCB)
- Are you sure you are going in and out on the correct pins of your rotary switch

There isn't much to go wrong with the caps.

 

[dacapitan]

Ok.  The aim is to isolate the problem (you could still have a faulty or mis-wired pot but it's not your only issue).

If your 1k resistor is good we can now say for reasonably sure that the issue is in your inductor/switch/cap combination.

Jakob makes a good point about DCR, what inductor did you use?

Things to double check:
- Are you sure you are going in and out on the correct pins of the inductor (pinout may not match your PCB)
- Are you sure you are going in and out on the correct pins of your rotary switch

There isn't much to go wrong with the caps.

Thanks for your help ruairioflaherty and Jakob - and for making these projects possible, much respect!

For some reason the attachment feature keeps giving me an error (with all files/all sizes) so I have uploaded the inductor specs here: https://www.sendspace.com/file/fqh8rt

I am sure they are mounted and wired correctly. When checking the inductors again earlier, I accidently jumped from the Low Boost Inductor input to the Mid Cut input pot , as I thought it was the low boost pot, there is definately a jump in the audio there, but nothing when using the Low Boost 2.5K pot - I dont know if this helps?

(this is the pot I used)
http://za.mouser.com/Search/ProductDetail.aspx?R=RV24AF-10-15R1-B2.5K-3virtualkey14860000virtualkey31VA303-F3

 

[ruairioflaherty]

The key to solving the problem is to understand want the circuit is doing in that section.

A "passive" EQ like the MEQ uses one or several voltage dividers to drop the level, these are placed before a make up gain amplifier  which brings the overall level back up to unity.

In the case of the low boost in the MEQ it's as Jakob explained, the 2.5k pot and 1k resistor to ground are creating a voltage divider and level loss.  The wiper of the pot allows us to lower the series resistance that feeds our filter to a varying degree, in effect allowing our filtered signal to partially bypass the level drop induced by the voltage divider described earlier.

The filter is just an inductor feeding a cap, complicated by the fact that each is selectable.  The inductor reactance is creating a varying impedance for the highs and at its simplest is rolling them off like a low pass filter, the smaller the inductance the higher that cutoff.

The cap is doing the opposite, rolling off the lows and the bigger the cap the lower the cutoff.  Combine the two and we have a nice bandpass filter.

Once you can understand the elements troubleshooting this will be much easier.

To see how the varying voltage divider works and test the pot use an alligator lead to short out the wiper with the output (as the circuit sees it.  As you vary the pot you should get an overall boost in gain?

- While we're on the pot are you sure it's as 2.5k?
- Also, are your overall levels correct into and out of the unit?  Is it working at roughly unity gain?

More soon, the baby's crying

Ruairi

 

[dacapitan]

The key to solving the problem is to understand want the circuit is doing in that section.

A "passive" EQ like the MEQ uses one or several voltage dividers to drop the level, these are placed before a make up gain amplifier  which brings the overall level back up to unity.

In the case of the low boost in the MEQ it's as Jakob explained, the 2.5k pot and 1k resistor to ground are creating a voltage divider and level loss.  The wiper of the pot allows us to lower the series resistance that feeds our filter to a varying degree, in effect allowing our filtered signal to partially bypass the level drop induced by the voltage divider described earlier.

The filter is just an inductor feeding a cap, complicated by the fact that each is selectable.  The inductor reactance is creating a varying impedance for the highs and at its simplest is rolling them off like a low pass filter, the smaller the inductance the higher that cutoff.

The cap is doing the opposite, rolling off the lows and the bigger the cap the lower the cutoff.  Combine the two and we have a nice bandpass filter.

Once you can understand the elements troubleshooting this will be much easier.

To see how the varying voltage divider works and test the pot use an alligator lead to short out the wiper with the output (as the circuit sees it.  As you vary the pot you should get an overall boost in gain?

- While we're on the pot are you sure it's as 2.5k?
- Also, are your overall levels correct into and out of the unit?  Is it working at roughly unity gain?

More soon, the baby's crying

Ruairi

I managed to get it solved! - I had the SRPP board OUT wired to the filter board IN - needs to be OUT - OUT and IN - IN - low boost working fine now. Thank you so much for all the help I really appreciate the efforts! I hope this will help anyone who has the same problem as me in future!

 

[ruairioflaherty]

Excellent, well done.