AKG C451E Upgrade Suggestions

[gabasa]

I've been looking at the schematics for the vintage AKG C451E and it looks like it would be pretty simple to upgrade; all it would need is up to four component swaps. Have a look at the attached drawing, I think that if I replace C1, C2, T1 and T2, it would be all this mic might need. When Jim Williams did his C451/C452 mods, he left the transformers intact and told me that he thought that they were great. These are my thoughts:

• C1: Replace with an Amtrans AMCH copper-foil 470pF
• C2: with a little Elna Silmic II cap of the same value, and bypass it with a little Amtrans AMCH copper-foil 1000pf on the bottom of the pcb.
• T1: Leave it alone.
• T2: I have some old Hitachi C2545 transistors in my box to replace the BC183B that's originally there.

Here are the questions in my head:

• Does C2 need to be as big as 33uf? For example, the Neumann KM84 uses a 1uf in the signal path right before the output transformer. Why did AKG choose 33uf?
• Any suggestions to replace the 2SK30R that AKG used for T1? Would you leave it as-is?
• Would my C2545 be a good substitute for T2? What would you suggest?

Thanks so much for the help. It would be fun to rework one of two mics in a pair, then make comparison recordings! Cheers and thanks.

 

[kingkorg]

I've been looking at the schematics for the vintage AKG C451E and it looks like it would be pretty simple to upgrade; all it would need is up to four component swaps. Have a look at the attached drawing, I think that if I replace C1, C2, T1 and T2, it would be all this mic might need. When Jim Williams did his C451/C452 mods, he left the transformers intact and told me that he thought that they were great. These are my thoughts:

• C1: Replace with an Amtrans AMCH copper-foil 470pF
• C2: with a little Elna Silmic II cap of the same value, and bypass it with a little Amtrans AMCH copper-foil 1000pf on the bottom of the pcb.
• T1: Leave it alone.
• T2: I have some old Hitachi C2545 transistors in my box to replace the BC183B that's originally there.

Here are the questions in my head:

• Does C2 need to be as big as 33uf? For example, the Neumann KM84 uses a 1uf in the signal path right before the output transformer. Why did AKG choose 33uf?
• Any suggestions to replace the 2SK30R that AKG used for T1? Would you leave it as-is?
• Would my C2545 be a good substitute for T2? What would you suggest?

Thanks so much for the help. It would be fun to rework one of two mics in a pair, then make comparison recordings! Cheers and thanks.

• View attachment 125886
• 
  
• for T2?

I personally analyze (measure) the whole thing in order to find if it can be improved on, see if there are any bottlenecks. I like to believe we are past the era of just swapping components for "better" ones. C2 is in low impedance path, using a smaller one would limit low end response.

Why replace 2sk30? It is sought after fet, known to be excellent performer. The whole circuit seems to be very simple, unity gain type that should take lots of signal with low distortion. Not much "character" coming from any component besites maybe the transformer. Much different circuit than km84, which picks up signal from FET's drain. Check out difference between common source vs common drain circuits.

I personally don't believe capacitor type or grade impact the sound as long as the right one for the job is chosen, and the stock one is not misbehaving. Never heard, or measured any difference. But as i know many disagree, feel free to experiment. Just take care you use the right type for the job so you don't get issues that might not be obvious right away.

 

[Rob Flinn]

The whole circuit seems to be very simple, unity gain type that should take lots of signal with low distortion.

Not in my experience. I find if I use these anywhere near a drum kit I need my screw in pads or the head amp in the mic clips pretty badly.

 

[Gus]

I have not worked on a 451 but looking at the schematic changing parts will most likely make it worse

The amp is powered at 9VDC that is most likely why it clips. With a circuit like this you need to think about Vdg and leakage,.
I like to reduce the capsule voltage instead of using a cap pad in microphones
It looks like the capsule operates around 60VDC the lower voltages at the 4.7meg are from the loading of the meter used to measure the voltages
Maybe making R7 10meg and adding a swichable 4.7meg from the R7 C6 junction to ground to reduce the voltage to about 1/3 of the orginal charge voltage will help with clipping. Now this will only work if the DC to DC can supply enough current

 

[kingkorg]

Not in my experience. I find if I use these anywhere near a drum kit I need my screw in pads or the head amp in the mic clips pretty badly.

I have not worked on a 451 but looking at the schematic changing parts will most likely make it worse

The amp is powered at 9VDC that is most likely why it clips. With a circuit like this you need to think about Vdg and leakage,.
I like to reduce the capsule voltage instead of using a cap pad in microphones
It looks like the capsule operates around 60VDC the lower voltages at the 4.7meg are from the loading of the meter used to measure the voltages
Maybe making R7 10meg and adding a swichable 4.7meg from the R7 C6 junction to ground to reduce the voltage to about 1/3 of the orginal charge voltage will help with clipping. Now this will only work if the DC to DC can supply enough current

Now that's GDY awesomeness at play!

Weirly enough, i remember these being pretty popular as overheads, hi hat mics.

Could it be the transformer is the weak link after all? Other than that i can't see why it would clip that easily.

 

[OneRoomStudio]

Technically, what you attached is the schematic for the C451 (no “E”). The “E” and “EB” versions are slightly different. I don’t think a few capacitor or transistor replacements will improve these mics at all.

 

[OneRoomStudio]

FYI, for those interested, I just added the manual for C451E, C451EB, and C452EB to the technical docs section.

 

[gabasa]

Thanks for all the info. Perhaps it would be a better idea to just get another C460B and do the Jim Williams mod to it. I know I'll be happy with it in the end.

 

[k brown]

I find it difficult to believe that anyone would be unhappy with a stock 451.

 

[rock soderstrom]

IMHO, capacitor mods are definitely underrated here on Groupdiy. I've made it a habit to do nothing else for a whole day once a year. I always do it on April 1st, which brings extra sound benefits, I don't know why, but it does!

Here's my current project, just finished. A pair of AKG 414 B-UL. Now they sound really good


BTW: check my thread @ Lab, interesting news from Hintertal!

 

[kingkorg]

IMHO, capacitor mods are definitely underrated here on Groupdiy. I've made it a habit to do nothing else for a whole day once a year. I always do it on April 1st, which brings extra sound benefits, I don't know why, but it does!

Here's my current project, just finished. A pair of AKG 414 B-UL. Now they sound really good

View attachment 125943View attachment 125944
BTW: check my thread @ Lab, interesting news from Hintertal!

 

[Gavin Pursinger]

I have the 20db screw in pads
That akg made to go between
The body and capsule.
I NEVER REMOVE THEM.
MY PAIR OF OLD CE451s are exceptional stereo pair that need NOupgtading.maybe something else is wrong with yours ?

 

[Rob Flinn]

I have the 20db screw in pads
That akg made to go between
The body and capsule.
I NEVER REMOVE THEM.
MY PAIR OF OLD CE451s are exceptional stereo pair that need NOupgtading.maybe something else is wrong with yours ?

I have a pair of 451 & another pair of 451eb. I keep 10db pads always on the 451EB.

 

[Rob Flinn]

Now that's GDY awesomeness at play!

Weirly enough, i remember these being pretty popular as overheads, hi hat mics.

Could it be the transformer is the weak link after all? Other than that i can't see why it would clip that easily.

When I first got my 451's I used to use them for overheads, but couldn't work out why the signal was always clipped regardless of mic amp level etc. It's not subtle when you look at the waveform. I then worked out that the mic itself was clipping. No issues with the pads inserted. Maybe that's the reason the pads go for so much money now.

 

[kingkorg]

When I first got my 451's I used to use them for overheads, but couldn't work out why the signal was always clipped regardless of mic amp level etc. It's not subtle when you look at the wavedform. I then worked out that the mic itself was clipping. No issues with the pads inserted. Maybe that's the reason the pads go for so much money now.

I guess the pad is just a cap in parallel with the capsule. Could be a great and simple DIY project to make these pad inserts. It still baffles me why they clip.

 

[Voyager10]

Some measurements or waveform captures would be useful.

It's not the transformer clipping, is it? There's going to be a lot of DC current ithrough the secondary if it's pulling a 48V phantom supply down to 9V.

 

[kingkorg]

There's going to be a lot of DC current ithrough the secondary if it's pulling a 48V phantom supply down to 9V.

Nice catch, haven't thought about this!

 

[terry setter]

Here's my current project, just finished. A pair of AKG 414 B-UL. Now they sound really good View attachment 125943View attachment 125944

And, they're so much harder to drop because of all that extra gripping surface!

 

[CalavoBob]

My C451s always sound better after I store them in a sealed Mason jar with silica jel desiccant. Same with my vintage C414s.

 

[CalavoBob]

I also always use the screw on pads if they're near a drum set. These are such great mics.