Repairing Rane HM42 Headphone Amp - Page 1
Forum section: electronics / projects

author: alleykatz
author_rank: Newbie
timestamp: November 27, 2017, 10:25:25 am
content: Almost completed with recap and pot replace and Deoxit. Have decided to replace summing opamp NEC C4570 with NE5532. I'd like to replace the line opamps (NEC C4072) but I can't find any info on them. They're from the late 90's I think. Anyone know of a suitable replacement for the NEC C4072? Thanks in advance.

author: bobaruni
author_rank: Regular Contributor
timestamp: November 27, 2017, 12:11:26 pm
content: Here is the datasheethttp://datasheet.octopart.com/UPC4072C-NEC-datasheet-101880.pdfThere are many FET OPAs that will be an improvement on the NEC part.Best bang for buck for FET input OPA IMO is perhaps the TI OPA2134.

author: alleykatz
author_rank: Newbie
timestamp: November 27, 2017, 11:11:55 pm
content: Great! Thanks for the confirmation. Found the 4072 just after I posted and looking at the schematic, Rane stated that an equivalent substitute could be the TL072. I did some research and discovered the OPA2134 (an upgrade for the TL072) as you have suggested. The specs show improved noise and slew rate and though the 4072 sheet doesn't specify THD, I suspect it will be better.There are four of them as line amps for the 4 inputs (dual-mono unbal). I found 5 for $14US from a US vendor but the picture and description show them as "original BB" with the BB imprint on the chip. Do these earlier chips differ from the later TI units in their specs? If so, should I be looking for these later TI chips?

author: Someone
author_rank: Super Contributor
timestamp: November 28, 2017, 06:24:07 am
content: Quote from: bobaruni on November 27, 2017, 12:11:26 pmEven TI recommend the newer and cheaper OPA1652, the OPA2134 is old hat and was never good value.

author: bobaruni
author_rank: Regular Contributor
timestamp: November 30, 2017, 09:06:27 am
content: Quote from: Someone on November 28, 2017, 06:24:07 amI understand that the OPA1652 is newer and cheaper, that doesn't mean it's suitable as a replacement, remember that it's not made in a DIP package.I compared the lowest noise and distortion OPA's regardless of the package, manufacturer or cost by means of circuit simulations and in real life and both to my ears and in the simulations, the OPA2132/2134 were pretty good but characteristic of that Burr Brown dark sound.The OPA1652 was very harsh because it went into early overload due to poor output swing so I would not recommend it unless you had enough power supply (more than +- 6V).

author: Someone
author_rank: Super Contributor
timestamp: November 30, 2017, 10:39:54 am
content: Quote from: bobaruni on November 30, 2017, 09:06:27 amThe product the OP is working on has freely available schematics:http://www.rane.com/pdf/old/hm42sch.pdfVery hot supply rails, and no strong requirement to stick strictly to FET input opamps.

author: bobaruni
author_rank: Regular Contributor
timestamp: November 30, 2017, 12:56:19 pm
content: Yeah OK, after looking at the schem, agree no need for FET...I suspect some fast OP amps might be unstable in this circuit, I would use an op amp that is unity gain stable at least and possibly of limited GBWP.

author: bobaruni
author_rank: Regular Contributor
timestamp: November 30, 2017, 01:12:48 pm
content: Quote from: alleykatz on November 27, 2017, 11:11:55 pmSome say that OP amps made at different fabs and times can sound different and although that's possible, I doubt that I could hear that difference in a blind test.There are fake versions of many ICS out there, I tend to think people would less likely bother to fake an OPA2134 but are much more likely to fake an OPA627.