MeToo2
Well-known member
Final photos after mounting everything up: I may still get some brass /etched labels done for the controls.
"Another Poor Man's" Fairchild 660/670
- Thread starter rotheu
- Start date
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Kingston
Well-known member
What a monumental build! I'm in awe. You are an inspiration to us all.
![[silent:arts]](https://cdn.imagearchive.com/groupdiy/data/avatars/m/0/172.jpg?1620371656){kind=avatar}
[silent:arts]
Well-known member
Impressive. Congratulations!!!
MeToo2
Well-known member
Please don't forget to thank the thread starter, Rotheu, too. He was my inspiration.
Control voltages go down to well over -60V. That cuts off the signal amp to just a few mV of bias very quickly to give about 15dB maximum of compression. It doesn't seem to clip until over 35V peak to peak (+24dBm) as far as I can see, which gives a headroom of around +20dB. Gain is around 7dB. Noise floor is around 63dB - 66dB down relative to +4dBm out whilst compressing, higher when not compressing.
I think that it's pretty quick on the attack times. Not sure what method they originally used to measure the 0.2mSec specification for the attack time, because there's no standard for this as far as I understand.
I measured it using bursts of 0dBm sine wave tones (2.178V peak to peak) at various frequencies (100, 1000 & 7350 Hz) with gaps between them to allow the release to recover a bit and then examining the output waveform.
I reckon it took about 2 1/2 to 3 cycles at 7.35KHz = roughly between 0.35 to 0.40mSec to get to 63% of the original level. For a square wave or a bigger transient then it seemed to be slightly quicker. This is perhaps the trade off I made for slightly lower bias currents on the ecc99. Or perhaps it's just the difference in measurement technique.
Release times were very long on the higher settings. Many seconds. Again difficult to quantify exactly although I could give it a go.
Position 4 was around 5 seconds, which matches the 660 spec.
As for the "sound" I have to say it's pretty quiet and pretty neutral so far. And it hasn't overheated or gone unstable whilst handling any techno.
lewilson said:
Control voltages go down to well over -60V. That cuts off the signal amp to just a few mV of bias very quickly to give about 15dB maximum of compression. It doesn't seem to clip until over 35V peak to peak (+24dBm) as far as I can see, which gives a headroom of around +20dB. Gain is around 7dB. Noise floor is around 63dB - 66dB down relative to +4dBm out whilst compressing, higher when not compressing.
I think that it's pretty quick on the attack times. Not sure what method they originally used to measure the 0.2mSec specification for the attack time, because there's no standard for this as far as I understand.
I measured it using bursts of 0dBm sine wave tones (2.178V peak to peak) at various frequencies (100, 1000 & 7350 Hz) with gaps between them to allow the release to recover a bit and then examining the output waveform.
I reckon it took about 2 1/2 to 3 cycles at 7.35KHz = roughly between 0.35 to 0.40mSec to get to 63% of the original level. For a square wave or a bigger transient then it seemed to be slightly quicker. This is perhaps the trade off I made for slightly lower bias currents on the ecc99. Or perhaps it's just the difference in measurement technique.
Release times were very long on the higher settings. Many seconds. Again difficult to quantify exactly although I could give it a go.
Position 4 was around 5 seconds, which matches the 660 spec.
As for the "sound" I have to say it's pretty quiet and pretty neutral so far. And it hasn't overheated or gone unstable whilst handling any techno.
MeToo2
Well-known member
They're actually just cheap meters from eBay user "jims_audio" from Hong Kong with retro styling & costing 10 euros or so each. More for show really. They are currently wired to give an indication of gain reduction (tap off the cathode resistor of the signal amp.) Apparently the original 660 also didn't have true VU meters either as the diode was removed. They seem to be pretty useful for detecting the onset of compression (coarse setting of the DC and AC threshold on the control amp) but any fine tuning means using your ears. I have a 4 pole 2 way switch that I'm thinking of adding later to allow me to switch to monitor output level, but they're almost certainly not true ballistic VU meters so I'm not sure how useful that'd be (still need to test). I found a standard multimeter and a high voltage oscilloscope across one of the screen grid stopper resistors was as good a way as any for checking for bias balance on the signal amp.syn said:
Here's the current eBay buy it now auction http://cgi.ebay.nl/ws/eBayISAPI.dll?ViewItem&item=220537048193
SUPERMAGOO
Well-known member
someone comes up and adapt to these circuits pcc189/7es8?
are very cheap tube. are used in the ear660
review ear660: http://www.gearslutz.com/board/high-end/71447-ear-660-users-id-love-compare-thoughts.html
http://soniccircus.com/EAR-660
are very cheap tube. are used in the ear660
review ear660: http://www.gearslutz.com/board/high-end/71447-ear-660-users-id-love-compare-thoughts.html
http://soniccircus.com/EAR-660
I never try to make my builds look like the units they are based on, but this time I went for original look as close as I could. The original Fairchild 670 just looks too cool not to try to recreate it.
They are some changes to the design, but I'm not sure if I should post them, there was not much interest in this topic. Sorry for crappy pictures, my camera sucks.
They are some changes to the design, but I'm not sure if I should post them, there was not much interest in this topic. Sorry for crappy pictures, my camera sucks.
haima
Well-known member
awesome! thanks so much for sharing rotheu - hopefully one day i'll get around to building one!
Matthew Jacobs
Well-known member
Rotheu... You're the man!!!!
Thanks so much for sharing... There might not be much interest, but the interest you are getting is from die hard fans like me ;D I've also been working on a unit for time and welcome these updates, thanks so much.
What a beautiful build.... It is awesome!!!!! Well done...
How's it sounding? Did you have any buzz and hum issues with your build? I'm planning to build a unit exactly like that... P2P... direct soldering to valve sockets etc... Is there anything I need to look out for?
Much respect!
JD
Thanks so much for sharing... There might not be much interest, but the interest you are getting is from die hard fans like me ;D I've also been working on a unit for time and welcome these updates, thanks so much.
What a beautiful build.... It is awesome!!!!! Well done...
How's it sounding? Did you have any buzz and hum issues with your build? I'm planning to build a unit exactly like that... P2P... direct soldering to valve sockets etc... Is there anything I need to look out for?
Much respect!
JD
noulou
Well-known member
Matthew Jacobs
Well-known member
Rotheu, I have a few questions...
Are those knobs at the bottom of the unit the DC threshold?
Why did the original unit not have them on the face plate? Or have I missed something?
They control the ratio right?
On the schematic of the original 670 it looks like they are trimmers, is that the case?
Are you feeding the plates of the 6BC8 direct from the 240V supply? IIRC this is over their rated speck. Why not 100V anymore?
Why didn't you use a stepped attenuator for the threshold? Wouldn't it help with stereo tracking?
Thanks in advance...
Cheers
JD
Are those knobs at the bottom of the unit the DC threshold?
Why did the original unit not have them on the face plate? Or have I missed something?
They control the ratio right?
On the schematic of the original 670 it looks like they are trimmers, is that the case?
Are you feeding the plates of the 6BC8 direct from the 240V supply? IIRC this is over their rated speck. Why not 100V anymore?
Why didn't you use a stepped attenuator for the threshold? Wouldn't it help with stereo tracking?
Thanks in advance...
Cheers
JD
