GML 8302/04 WOW!

[Peter Simonsen]

afaik it's a simple trsansformerless one opamp circuit, no surprises in the basic design. The secret ingredient is the opamp... It might be interesting to discuss the circuit and guess why the opamp suits the task so well or why it sounds so good. Maybe there is a suitable opamp availble nowadys? I don't have any confirmed information on the original opamps I only know  that they are potted and like to explode once in a while.

Michael

Well..out of love & respect to George all I will tell you is that there is nothing "simple" about the design/circuit. And No its far from just a "simple one opamp design". The GML all  discrete transistor class-A transformerless mic-pres are imho among the very best preamps if one is after a preamp that just amplifies the mic and source. I love them and has used them since ohh early 2000´s.

The same goes to the GML 8900 compressor seriesII..sadly Out of production for couple of years as I love that one. The GML 8900 can be looked at..as 3 compressors "in one" per side of the stereo unit.  Many have had a hard time "understanding" the 8900. I never understood this. Its quite logical imho. Okay!.. Its a bit more complicated than a LA2A, 1176..but really not much.  One hell of a compressor, that offers DRC that beats the living daylight out of anything elese imho.

His digital plugin DRCII is a GML 8900 on steroids.

Kind regards
Peter

 

[Kingston]

gearslutztalk without a clue

This really does not belong here.

In fact most of this thread does not. There not a single trace of technical ground or specifications to walk on. Let alone design which we could talk about at length.

 

[alhe]

Wow, that is quite an amp module. Notice there is a standard at opamp at the heart of it. Low noise transformers at the front end, ala SSL maybe, and some for extra current drive as well? I have no idea really.

 

[paolomunarini]

Hi knotty. Could you please send me a photo of the small vertical PCB on the top left corner?

I'm repairing one unit and I'd like to know the value of the two resistors placed on that small board.

Somebody shorted out the phantom power and the current limiting resistor is blown...

Edit: it looks like the resistor somehow "resisted" (pun intended) to the short, but the DC DC that produces the 48V is gone. I need to find a replacement now... Wish me luck...

 

[musgrave]

When George moved to Nashville, he closed his lab and assembly line. Jose, Manny and others went to Manley and EveAnna did assembly for a while. Now the assembly is back in george’s world.

 

[Toobme]

The GML pres are amazing, and as someone asked, we preferred them for acoustic jazz, but usually grabbed the Forrsell for our classical gigs. That and the HV3D put into an EMM Labs A>D doing DSD. Got some pretty “real” sounds with all of them, but yes, the GML has a kind of signature I can’t describe but quite enjoy. Sorry, no tech info from me on this one - I’m now REALLY curious about the opamp in there. I wish I still had access to it! Somebody can hopefully post a closer-in pic of the guts.

 

[Dylan W]

Yeah they definitely do! I had to lug in a large power supply with it haha

I did read somewhere that they were built by Manley too. Which is kind of interesting. Although I know a ton of history about lots of companies, GML is like a black hole to me as far as knowledge.

I did find one picture that looks like it has quite the opamp on the front end. I honestly don't think I've ever seen an opamp with that complex of a circuit on it. No wonder it sounds so different haha

Venturing a guess at topology...

The scary looking discrete amp in the front looks like a dual op amp - if you take a careful look the layout is mirrored.

Probably the pre is probably a Cohen topology.

So, deducing, maybe the 8 pin smd device on the dual amp block is an SSM2212 or similar, forming the transistor pair front end (in conjunction with the dual op amp). That would mean the little amp block at the back could be the 3rd op amp in the Cohen topology. Maybe the same op amp as the dual amp block, but with TO92 devices in the output stage for (slightly) better power handling.

I have a schematic for a GML op amp. I think I got it here, should be floating around. Will avoid posting it as the product is still in production, but it's a reasonably normal-looking 3 stage design. Probably pretty fast. Of note: a pair of transistors for the VAS, TO92 devices in darlington pairs for the output stage, several flavors of internal compensation.

 

[Dan Kennedy]

It's not Cohen, it's a three amp instrumentation configuration. Used to be LM394 for input devices, would suspect later versions use the SSM2212 or some other similar low noise bipolar pair. First two amps are somewhat different than last stage. But I'm going from a 1980's vintage I explored back then.

 

[Dylan W]

It's not Cohen, it's a three amp instrumentation configuration. Used to be LM394 for input devices, would suspect later versions use the SSM2212 or some other similar low noise bipolar pair. First two amps are somewhat different than last stage. But I'm going from a 1980's vintage I explored back then.

Good info Dan. Do you mean instrumentation amp with transistors in front in current feedback? (Like Amek 2500, etc.) Or is it just straight into the op amps? The configuration with transistors in front is what I meant by Cohen topology, though I guess strictly speaking the Cohen topology has a double balanced output

 

[Dan Kennedy]

I'd have to go way back in notebooks, and it's not my IP to give away, but the topology I remember is basically the two input opamps were current feedback, with integral low noise devices, not a good opamp with a low-noise tacked on front. But again, memory from almost 40 years ago.