| Author | Message | ||
     Kevin McCready |
I am attempting to completely redesign the preamp section of my friends HD-130 bass amp(2100-130). I am using Burr-Brown OPA-627's and have built a new power supply from a design of Walt Jung and Gary Galo. I am using all premium parts because this amp was given to him and it needed new tubes anyway.I would like to know if anyone out there has any suggestions on changes to the preamp siginal path to improve bass response, transient response etc.I wonder if I even need coupling caps with the OPA627's extremly low bias current. Also can anyone tell me what the double 1N914 diodes do on the input of the first LM307. I would like to optimise this circuit for the OPA627's and take out some of the bandaids that were originally designed for the ancient LM307's .I talked to an application engineer from National about replaceing the LM307 and he asked why would you want to,the present day op-amps are light years ahead of what was available in 1965. Any number of op-amps available from a number of manufactures today are actually almost tube like in sound.By the way 8pin TO-8 to 8pin dip adapters are available from Digi-Key.(A330-ND)Any input on this matter would be much appreciated because I have searched the web and this site is like a gold mine for Music Man owners. | ||
| DaveEmminger |
The back to back diodes across the op amp inputs form a simple amplitude limiting clamp of one PN junction (about .6 volt for silicon). That's my best guess. | ||
| Kevin McCready |
Dave, I think your correct when you say the double diodes in parallel with reversed polarity to the inverting and non-inverting inputs of the first op-amp of this preamp is some kind of clamp,and I think I remember seeing this set-up in one of my electronics books 20 years ago, but what is it doing is this circuit and can I just remove them . I am useing Burr-Brown OPA-627's to replace the LM307's and maybe they are not needed in the circuit. Maybe the LM307H's were prone to overloading. Any comments would be welcome.  | ||
| DaveEmminger |
Kevin, Yes, you can remove them, and then, OTOH, there's really no need to, as the clamp voltage of .6 volts is way above signal level at that point in the circuit. I think they're for protection. | ||
| Lee_Ranaldo |
Back to back diodes in the Non inverting and inverting input in pre-amp desings are for protection to extend the OP-AMP life. If you want to add a distortion stage just put a pair of back to back diodes from the ouput of the IC (pin 6 i don't remember right) to the input. you can make a tubescreamer inside the preamp of this model. (very nice), or simply emulate various preamps models using op-amps. I've own a blues driver that it's already modeled to emulate the tone circuit of a fender. so a good choice is to add a normalized LOOP in this amp, it's so easy. | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
I checked on these adapters, they are $6.05 each. at Digikey. | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
Also you can get the Brown Dog adatpors for $2.75 ea. at this time. | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
Reed Ohrbom and I have been researching some IC replacements for the older LM-307H and are at this time working on getting some to try out. The OPA-627 that Kevin Mccready had installed in his amp are high on the list, but very pricey! They are almost as expensive as a NOS Telefunken 12AX7 smoothplate, but will outlast and perform circles around the Tele, so they say (A tube is still a tube and a simiconductor will always be a simiconductor trying to replace tubes)and only need to be installed once for the life of the amp. The OPA-627 is regarded on www.diyAdio.com as the best sound, dynamic, closest to a tube as one can get. The AD825 chip is also a good candidate. It may be somewhat expensive to upgrade to this chip, but maybe worth it to some. There are other less expensive chips available for pennies. In some cases a LM1458 will sound way better to some players who want a sweet, slightly less gain in the bass and high treble frequencies. Where the better performing LF353, TLO72 type's will be better performing spec-wise, maybe for cleaner, fuller preferences. Its like trying to compare a a Telefunken smoothplate to a Ei smooth plate. The two look almost identicle, maybe Ei uses the old Telefunken tooling, but there are some differences in tone and microphonics. I have done comparisons on a NOS Telefunken and the Ei. Well the Tele was made many years ago when the Germans used very high quality components and a rigid production control. The Tele was guaranteed to last some 200,000+ hours of life, and many are still running strong today. They have a very smooth detailed sound, but may be too smooth for some folks. Now as for the Ei grey plate that I have tried, they seem somewhat inconsistant in performance unless they have been prescreeened for microphonics and gain. They do sound very good in some amps especially in the overdrive/distortion section. Nice midrange, bite and attack, but get michrophonic and die quick but are fairly inexpensive. I like the sound in some applications, say a Boogie Mark I or Marshall. The Ei usually have more gain and bite than the Tele's that I compared them to. The Sovtek 12AX7 LPS is another one that has a decent sound stage, is full, has a good gain etc, for a modern day production 12AX7, and I use one in my D*mble-like clone that I built, in the first stage. In that amp it works very good. The second stage or overdrive stage, I always go back to a NOS Raytheon blackplate for a very nice, clear sweet Backface Fender type overdrive for Blues/Rock tones, and in the driver stage I love the GE6681, its an Industrial version of the 12AX7, but this one has some gain goin on! Full, great soundstage, and hard to get sometimes since its Old stock. Where I'm going with this is to try combining chips like 12AX7's may find one the tone they are searching for, though it may be subtle, everything you can do adds up to a result. I hope this helps some fo you looking to improve the sound of their amp. We are looking into adaptors for the T0-99 (can type chip)to adapt modern chips for easy replacement for the HD-130 tthough if you want to spend the extra money for a military grade can type, your looking at about $50+ for a OPA-627 which is supposed to be comparable to a very good 12AX7 and never should need replacing as any chip... Some prefer the NE5534 (dual opamp) for getting close to a Mullard 12AX7 tone, it it is similar and much cheaper than the OPA-627, a LM1458 is sweeter mids, less bass-gain and top end-gain than say a TL072, so it all a matter of taste. Ed | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
The NE5532 works great in RD-50's for more warmth and gain like that of a Mullard. In the HD or 12ax7 pahse driver amps, with the LM307H IC's, a an adaptor with a LM356 works great! better than the expensive IC's and a lot cheaper than the LM307, with better sonic quality without being harsh, very musical. |