drip fourseven HF Roll Off

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gg85

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Hey guys :)

I built this pre about 10 years ago, and it wasn't until recently that I ran a sweep through it to see the frequency response, which starts to roll off from about 5k until it's down 3dB at 20k.

I don't know if it's always done this, but I've tried new tubes, I've bypassed the input attenuator so that the signal is going straight into the input transformer. I've traced 20k with a scope and I can see a small amount of loss right at the primary of the input transformer. Then it looks like more loss after the first tube and a little more after the second tube. I know it's flat out of the converter because when I watch a sweep on the scope there is no change in amplitude. When I do the same with the scope at the input and each gain stage of the pre I see it get smaller after about 5k.

Does anyone have any idea where I can look next?

Thanks! :)
 

rock soderstrom

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Hi, It is hard to tell what the problem is and if there is one at all without more information. Can you please post the exact schematic of your build? There are numerous variations of this circuit. Which transformers? Any MODs? What is your measurement setup? Input and output load?
 

gg85

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Hi, It is hard to tell what the problem is and if there is one at all without more information. Can you please post the exact schematic of your build? There are numerous variations of this circuit. Which transformers? Any MODs? What is your measurement setup? Input and output load?
I believe this is the schematic that the drip pcb follows. I've also attached the frequency response.

I had an input attenuator that sits before the input transformer, but that is now removed and the problem remains the same. drip also has a 20dB input pad that is before the input transformer. I have the on a relay and there is no difference whether the pad is in or out. I am not 100% I understand what you mean by input and output load, I am just running fuzzmeasure using a UAD apollo for I/O. I am using the sowter 9970 and 9980 transformers.

Thanks!
 

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radardoug

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That transformer input circuit looks funny, with the zobel connected to the transformer centre tap. That could well be doing the rolloff. Take the cap out to see.
 

rock soderstrom

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Thanks for the info. This schematic is certainly the historical basis of all the current REDD47 clones. The devil is in the details. The Zobel shown is a good example because that will probably be different on the Drip PCB because of the different transformers.

I'm sure Drip has made other changes, so the correct schematic would be important already. Does Drip actually sell PCBs without schematics? What is the problem?

About your frequency response measurement, that may well be based on an incorrect arrangement. It's relatively easy to measure nonsense and then chase ghosts.

So you should spend some thought here and work out the concept. The output impedances of the signal generator and the input impedance of the measuring device are relevant parameters.

A good starting point would be to measure the amplifier circuit without input transformer in DI mode, then you can trace the possibly existing treble drop.

Have fun!🤓
 
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My3gger

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As it was mentioned input's secondary zobel filter could cause problem with frequency response. Sowter 9970 was made as a replacement for this preamp, instead of CT only like in original it also has 34,5% tap. I'm not sure if Drip took care of this on pcb or docs. Do they have 34,5% tap for cases when secondary is turned around to function as primary? ~50ohm tap at the primary makes more sense to me because it can be used for passive ribbon microphones.
 

Winston OBoogie

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The schematic showing the secondary network connected to a centre tap in the original REDD.47 amplifier is incorrect anyway.

The terminal on the input transformer where the network is connected is simply that, a terminal. It's a convenient anchor point that doesn't connect to anywhere on the secondary winding or anywhere else.
 
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My3gger

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Thanks, i thought so after looking at original schematic although Sowter's 37,5% secondary tap is still a mystery to me. Sowter's measurement suggests gg85's input transformer is not wired correctly, if signal generator's output impedance is correct for this job.
 

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Winston OBoogie

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if signal generator's output impedance is correct for this job.

That's a very important point right there (y)

A simple 40dB pad with a series R of 19K8 and a shunt of 200R will swamp out (or build out, whichever way you look at it) the impedance from the generator so that the transformer sees a typical mic impedance source.
 

gg85

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Primary connections look normal as per sowter’s wiring. Yellow and Brown are inputs. Centre tap is isolated. On the secondaries, grey is grounded, green is output, 34.5% is isolated. Zobel has been removed so the White CT is now also isolated.

No change in frequency response.

I also tried going straight into the DI input and there is a significantly worse roll-off when I do that.

I’m not too sure how the signal gen impedance could be incorrect for this measurement. I am just using the generator in Pro Tools. Output impedance of UAD Apollo is 600 Ohms. Input is 10k Ohms. I’ve checked other mic pre’s like this and haven’t had any issues.

I originally decided to check the response because I was using a Sonarworks mic to do a measurement of my room and noticed the HF was poor, but when I used a different mic pre it was fine. It had also been a little while since I used this mic pre and when I started using it again the roll off was more noticeable, maybe because the excitement of having finished the build wasn’t there and I was hearing it for what it is haha.

Drip doesn’t include schematics, it’s been a nightmare with other builds. Although he does tend to follow the classic schematic, just arranged differently. All cap/resistor values appear to be the same as the original EMI schematic, including that zobel.

I have a UTC A-10 spare, and I just wired it up so that my Apollo line out is going into the 50k side, and the 500ohm output feeds the redd. I then connected the in/out shields on pin 10. Doing this gets me something that I imagine looks correct, with a rise beyond 20k (see attached)

When I tried the A-10 with the hi-z output feeding the DI input of the mic pre, it still had a more significant roll-off than the original issue.

Really appreciate everyone’s help so far!!
 

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ruffrecords

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If the DI response is significantly worse this points to the amp and/or output transformer as the culprit. Looks to me like the input transformer is fine. There is obvious reason for HF roll of in this design unless you have a wrong value capacitor in the negative feedback loop.

Cheers

Ian
 

rock soderstrom

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If the DI response is significantly worse this points to the amp and/or output transformer as the culprit. Looks to me like the input transformer is fine. There is obvious reason for HF roll of in this design unless you have a wrong value capacitor in the negative feedback loop.
Yes, if the problem occurs in DI mode the feedback loop might have something to do with it. In which gain position are you measuring? Does the treble loss appear in all gain positions?
 

gg85

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Yep it occurs at every gain step. I’m going to check out C5 (1uf) and C2 (100uf) now and see if that might be my issue.

You guys are great, really appreciate the help!
 

rock soderstrom

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Yep it occurs at every gain step. I’m going to check out C5 (1uf) and C2 (100uf) now and see if that might be my issue.

You guys are great, really appreciate the help!
Better check C8=680pF as this is in the feedback loop for all gain switch positions if you haven't done so already.

The two capacitors you mentioned are more responsible for the lower cutoff frequency.

Check C7=330pF as well.
 

gg85

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All caps seem fine and are the correct values. I have replaced C8 and C7 but saw no change.
 

gg85

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I also just did a quick test with input transformer straight to output transformer and there’s no issue there. Just trying to eliminate anything I can think of...at this stage I’m a bit lost and there’s not really too much left in the path that I haven’t gone over. Also doesn’t look like there are any mods that drip has made that stray from the original schematic.
 

ruffrecords

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OK, maybe it is a function of your test set up. I know you have said it is OK with other mic pres but it is still worth checking. Can you describe how you are testing?

Cheers

Ian
 

rock soderstrom

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I also just did a quick test with input transformer straight to output transformer and there’s no issue there. Just trying to eliminate anything I can think of...at this stage I’m a bit lost and there’s not really too much left in the path that I haven’t gone over. Also doesn’t look like there are any mods that drip has made that stray from the original schematic.
I'm running out of ideas. I would also disconnect the output transformer and see if the naked amplifier also shows the behavior. A good idea would also be to trace the signal through the individual stages and see where the high frequency are lost. Be careful with your interface! HT!

Have you replaced the tubes?

How about an additional test with REW or RMAA?
 
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gg85

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Sure, I have a signal generator out of pro tools, line out from UAD Apollo (600 Ohms), into XLR in of mic pre. Output of mic pre into line in of UAD apollo (10k), and using q-clone to see the curve. I’m also using fuzzmeasure to do a frequency sweep and I get the same results.

I can hear the loss in HF when using a microphone as well, and when doing a sweep in Sonarworks of my room using a reference mic, there is significant HF loss. When I do the same sweep with another mic pre there is no HF loss.
 
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