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altec 1678c mod..direct out/gain control..AUDIO SAMPLES!
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RodC
dead but not forgotten


Joined: 31 Dec 2004
Posts: 2039
Location: Right outside the door

PostPosted: Tue May 06, 2014 8:55 am    Post subject: Re: altec 1678c mod..direct out/gain control..AUDIO SAMPLES! Reply with quote

RP wrote:
Hey guys,

This is some great info. I just ordered a 1678C and canít wait to try it out. I have a couple of questions though. According to the schematic, the 560 ohm resistor reduces the gain by 14db. and after you installed a 1K pot in itís place the signal was still too hot for most applications and needed an in-line pad to control. Is there a good solution for adding a switchable pad into this path? This may be an ignorant guess but if a 560 ohm resistor reduces the signal by 14db then the 1K pot should be the equivalent of a variable 25db pad. If you are still needing an in-line 20db pad then wouldnít a on/off switchable 800 ohm resistor to ground do the same thing?
Next, the phantom power is from what Iíve read mostly unusable at only 18V. Would it be possible to add a small step-up transformer in the power section that takes the +18V rail and makes it +48V. Would this affect the transistor(Q301) on the input transformer? Does it need to be changed out for the increased voltage?
The manual states that this unit has 90db of gain with the pad disengaged. How much of this is due to the output transformer and has anyone left one of the channels running through the output transformer to see what a difference it makes to the sound of the unit. Iím guessing the auto level control section on the PCB would not get used at all and could probably be removed all together. If you wanted to add an output transformer to each channel this seems like it would be a good place to mount it. Any thoughts?
Lastly, how would you make the outputs balanced and do you think it would be worth it?


I need to correct that article, I'm pretty sure that is a 10K pot I used, not a 1K. I have just been insanely busy lately.
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RodC
dead but not forgotten


Joined: 31 Dec 2004
Posts: 2039
Location: Right outside the door

PostPosted: Tue May 06, 2014 8:55 am    Post subject: Re: altec 1678c mod..direct out/gain control..AUDIO SAMPLES! Reply with quote

RP wrote:
Hey guys,

This is some great info. I just ordered a 1678C and canít wait to try it out. I have a couple of questions though. According to the schematic, the 560 ohm resistor reduces the gain by 14db. and after you installed a 1K pot in itís place the signal was still too hot for most applications and needed an in-line pad to control. Is there a good solution for adding a switchable pad into this path? This may be an ignorant guess but if a 560 ohm resistor reduces the signal by 14db then the 1K pot should be the equivalent of a variable 25db pad. If you are still needing an in-line 20db pad then wouldnít a on/off switchable 800 ohm resistor to ground do the same thing?
Next, the phantom power is from what Iíve read mostly unusable at only 18V. Would it be possible to add a small step-up transformer in the power section that takes the +18V rail and makes it +48V. Would this affect the transistor(Q301) on the input transformer? Does it need to be changed out for the increased voltage?
The manual states that this unit has 90db of gain with the pad disengaged. How much of this is due to the output transformer and has anyone left one of the channels running through the output transformer to see what a difference it makes to the sound of the unit. Iím guessing the auto level control section on the PCB would not get used at all and could probably be removed all together. If you wanted to add an output transformer to each channel this seems like it would be a good place to mount it. Any thoughts?
Lastly, how would you make the outputs balanced and do you think it would be worth it?


I need to correct that article, I'm pretty sure that is a 10K pot I used, not a 1K. I have just been insanely busy lately.
_________________
'Well, I've been to one world fair, a picnic, and a rodeo, and that's the stupidest thing I ever heard come over a set of earphones'

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RP
audio school


Joined: 13 Feb 2008
Posts: 3

PostPosted: Tue May 06, 2014 11:56 am    Post subject: Re: altec 1678c mod..direct out/gain control..AUDIO SAMPLES! Reply with quote

Im also a little confused about changing the location of the output control in the circuit. Rod suggested removing the connection between R136 and C108 then running from r136 to the center leg of the output pot. Gabreal removed R136 all together.
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Spark
suffering 'studio suck'


Joined: 17 Feb 2004
Posts: 414
Location: Ontario, Canada

PostPosted: Sun Sep 13, 2015 4:37 pm    Post subject: Re: altec 1678c mod..direct out/gain control..AUDIO SAMPLES! Reply with quote

[quote="RodC"]
RP wrote:

I need to correct that article, I'm pretty sure that is a 10K pot I used, not a 1K. I have just been insanely busy lately.


Hi Rod, did you ever have a chance to confirm it was a 10K pot that was used?
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Spark
suffering 'studio suck'


Joined: 17 Feb 2004
Posts: 414
Location: Ontario, Canada

PostPosted: Sat Nov 07, 2015 12:41 am    Post subject: Re: altec 1678c mod..direct out/gain control..AUDIO SAMPLES! Reply with quote

Im having a devil of a time figuring out what I did wrong with this mod. I am 100% sure I wired it up the same as described in this thread, but Im getting super low output thats distorted.

Being distorted it sounds like its hitting the input with more than enough level, but something is goofy on the output. My understanding is the existing knob that was there originally is the output level control. The only thing I can see that might be questionable is that I soldered the wire from r136 to the middle lug of that potentiometer as it sat. I dont have to disconnect that lug from the circuit before soldering to it, do I?

Thanks for any help or suggestions.
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Spark
suffering 'studio suck'


Joined: 17 Feb 2004
Posts: 414
Location: Ontario, Canada

PostPosted: Fri Nov 13, 2015 8:52 pm    Post subject: Re: altec 1678c mod..direct out/gain control..AUDIO SAMPLES! Reply with quote

Fixed it. Loose wires dont only cause fires, they also cause preamps not to have proper output. For the record I mean those two little wires that join each PCB.
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RP
audio school


Joined: 13 Feb 2008
Posts: 3

PostPosted: Sun Mar 20, 2016 10:09 am    Post subject: Changes? Reply with quote

The preamp circuit in this unit is very similar to the mic pre design shown in the Jensen schematics on their website. The main difference is that they use a reverse log, not log, 10K pot in the location of R103,203,303,403. This is the same type of taper used in most preamp designs. It may be worth even trying a 5K. I would toss the existing 10K log pot and make a direct connection between the output of the NE5534 and R136, bypassing the hi pass filter. No reason to use that, especially if you have to open the box to get to it. Another mod worth looking into would be copying the output on the Jensen AS017 mic pre schematic to create a balanced output. A 20db pad like the one on the Jensen AS016 schematic would be nice as well. The phantom power on this unit is practically worthless at only 18V and should be rebuilt all together if itís ever to be used. The power transformer already outputs 24-0-24 on itís secondary so copying the phantom circuit on John Hardyís power supply schematic would work but there are quite a few changes that would need to be made to the preís as well to get it to work properly. While youíre re wiring this thing you might as well swap out any electrolyticís on the preís and especially on the power supply. They are around 25 years old by now.
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