Altec Lansing 1678c- A $3000 Mic Pre for $100?

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rp3703

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Joined
Mar 23, 2014
Messages
47
Ok, I am very familiar with the TapeOp post on converting this mixer to an 8 channel pre, which is my goal as well but I want to make it even more versatile and thought it would be a good idea to get some perspective from this forum. For those not familiar with this unit, it is a potential goldmine of a preamp with four to eight channels of input transformer balanced preamps packed into a two space rack unit. It uses a single 5534A op amp chip that runs on +/-18V rails and that’s pretty much it. To me this puts it on par with an ISA-4 or an SC-1 or even a 312(minus the output transformer)which cost around $200-$400 a channel. You can listen to some samples of drums recorded through one of these modded units through a link on the TapeOp post.
The inside of the unit is divided into three to five cards depending on which model you have. A power supply card and either one or two sets of channel cards and input and output connection cards. Almost two thirds of the channel cards are populated by electronics that only serve the mixer functions of the unit and do not affect the pre amp portion of the circuit. Unfortunately the only gain attenuation takes place in the mixer section so the individual outputs have no volume control. This is where the TapeOp post takes off. They add a 10K(even though the post states 1K, both contributors told me that they think they settled on a 10K)log pot between R102 and C101. This is essentially the same circuit design as a 312, give or take a few component values. The TapeOp post suggests placing an output attenuator on it as well but I'm not entirely convinced that this is necessary(the 312 doesn't have it). I am convinced that the circuit needs a pad before the transformer though. I’m thinking of copying the pad that JLM uses for their BA preamp but I’m not positive of what value to use for the bridging resistor. I’d also like to take the +/-24V that is used to feed the power mute relays and use that instead to supply true 48V phantom power(The unit does have phantom but it is only 18V). The JLM site has schematics for a phantom power circuit that can be fed off of +/-24V and I found a pre-assembled voltage regulator on eBay for $10 that is, plus or minus a few parts and some rearranging, an exact copy of this circuit. The third thing I would like to do is balance the TRS outputs and maybe add a phase switch and a impedance selector. OK, enough. I’m self teaching myself all this stuff and I'm far from knowing the least bit of what I'm talking about. That's why I'm here. I don't know how to make sure all these things will work together. These mixers can be had for around $100 or so and my hope is with a little diy, they can be turned into a really hi-end mic pre. I don't plan to make any money with this and only want this post to help me and others repurpose their own units and save some bucks as well. There are endless DIY 312’s on the web these days but even those will run you around $150 a channel and you still have to come up with a case and a power supply. From what I can tell everything is already in a 1678, it just needs to be reconfigured a bit. Schematics for the 1678 can be found on the Altec Lansing history site which is linked on the TapeOp post. I’ll post some Schematics with the revisions I’m proposing as soon as I can. Thanks.
 

rp3703

Well-known member
Joined
Mar 23, 2014
Messages
47
Ok, here's my version of the schematic for the entire unit minus the parts that operate the mixer section. I've added the phantom power supply and pad that I had mentioned in my earlier post but I still need to figure out the value of the bridging resistor for the pad and how to balance the output. I'm hoping that the experts on this site can look this over and tell me where I've screwed up or placed redundant parts. Thanks.
 

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