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Ben HeadWize Fanatic Joined: Nov. 10, 2000 |  Message [#1] posted on: 01-09-2001 04:18 AM CST (US).   |  |  I am curious about whether I can drop in an OPA627 to replace an OPA604 in a preamp I hope to build. By extension, also curious about how these chips might interchange with the 132/134 family. Started this discussion at the bottom of a nearby thread, but hoping to attract some of you heavy-hitter circuit design types, hence this new post. I did some forum searches but they didn't help very much, so I downloaded some Burr-Brown spec sheets from Digikey Canada. The 132/34 family look virtually identical, as far as I can tell, although the spec sheets are organized differently to emphasize different things. Neither spec sheet shows the internal circuit topology of the chip. The 627 is FET all the way through, as opposed to the 132/134s which have FET inputs but bipolar outputs. You can put a 627 in the 132/134 socket, and the signal and voltage pins are OK, but there is something different going on inside, since one of the offset trim pins is in a different place (pin 5 instead of 8). The OPA604, another generally well regarded audio opamp, has an identical pin configuration with the 627, including the offset trim, and it is also an FET-input bipolar-output chip like the 132/134. Both the 627 and 604 spec sheets show the internal topology, and they are not even vaguely alike, even though the pinouts are identical and the specs pretty similar. On this basis I decided that the 604/627 drop in was probably not going to work, but I was just guessing based on jumping to quick conclusions from the spec sheet. And I am emphatically not a circuit designer. And don't know squat about opamps. Skippy says, in the other thread, that you can do drop-ins among these chips as long as you have enough power for the 604'2 and 627's. (Once again, reality triumphs over paralysis by analysis!) Anyone out there have more to share on this? Thanks! |
| Apheared HeadWize Fanatic Joined: N/A | Message [#2] posted on: 01-09-2001 04:58 AM CST (US). | | I'm not a heavy-hittin' designer by any stretch of definition, although I'm not the least bit apheared to blow sh*t up. :) Opamps in general are remarkably interchangable; specially when you're not dropping them into a already done circuit but building it yourself. So far the main thing I've found that differs is offset and bias currents. When building an amp with no output caps, you have to be VERY careful because you can have a circuit tuned for one chip, drop in another, and end up with VOLTS of DC at the outs (in which case, say buh-bye to whatever headphones were hooked up) It's not magic; it's easy to control by your resistor selections. Some chips are worse ("loose" I'll call them) and can vary by mA even with proper selections, you'll have to implement a nulling circuit to get them near 0VDC. Whereas some like the 132s are in pA offsets and you have to really try hard (or screw up) to get some DC at the output. The 2nd biggest thing is bandwidth. Some chips are real screamers, and like all high-performance things, you have to cater to them. Layout becomes extremely important! As does five or so compensation techniques to make them run stable. Datasheets will tell you most of this stuff; but take heart. You've got 3 things going for you already - it's not a pre-built circuit, so you can adjust accordingly. You're building an AC powered preamp, so you have plenty of voltage to drive the chip. And, GOD I hope, you will have output capacitors since it's a preamp - which stop that pesky DC dead in it's tracks. |
ppl Headphone Council Joined: Jul. 28, 2000 | Message [#3] posted on: 01-09-2001 10:22 PM CST (US). | | The OPA-627 is not an fet design throught. See the Schematic on the lower cover of the Data sheet. I see an FET long tail pair cascoded to Bipolars, With the second stage as well as outputs all Bipolar Transistors. |
| ThingyNess Headphone Council Joined: N/A | Message [#4] posted on: 01-09-2001 10:51 PM CST (US). | | ppl: I don't think i've EVER seen an "all-fet" opamp design, through all my time sifting through datasheets in the last few months. Many (j)FET-input designs, but VERY few use FETs throughout - I think I only saw one EVER that used a FET in the output stage. - Thingy |
| ppl Headphone Council Joined: Jul. 28, 2000 | Message [#5] posted on: 01-10-2001 03:46 AM CST (US). | | As i stated the OPA-627 IS NOT AN ALL FET OP AMP. I Made this point very Clear |
| Ben HeadWize Fanatic Joined: Nov. 10, 2000 | Message [#6] posted on: 01-10-2001 04:05 AM CST (US). | | Ooops, ppl, right you are. The FETs are the input pair only. Thanks! Sometimes I learn in private, sometimes in public... (Embarrassed smily face goes here.) OK, so now having learned to look more closely at circuit diagrams, let me re-ask the question. The topology of the 627 and 604 look completely different to me. How does that allow for drop-in? Does one ignore what's inside, and just make sure the pins are compatible and the input and output impedances in the same ball park? And what about the circuit topology makes the 627 cost an order of magnitude more than the 604? And why does it sound better? Thanks for your patience. |
| ppl Headphone Council Joined: Jul. 28, 2000 | Message [#7] posted on: 01-12-2001 07:06 AM CST (US). | | Generly you can forget the Topology of an opamp you are considering as a replacement with the following considerations. Supply voltage, Gain Bandwidth is verry importent because if you use a higher bandwidth part as the replacement as is the case with what you are doing then circuit layout will need critical attention. Unity gain stability is also importent for DIY'er who whant a good chance of proper opration. FET input Devices with FET input devices and most of the time BIpolar input Devices with Bipolar input types However an fet input can replace Bipolar inputs in most cases. The reverse is not often true because of input bias Currents. I think The OPA627 Will Replace The OPA-604 rather well and sound better. The OPA2604 is the Dual 604 and i Heard this part when it first came out I hated the sound of this part. Think the OPA-627 is light years ahead of the OPA-604 in sound. The Cost Difference is Because of DIFet Manufacturing process. Dielectric isolation is costly. I like the soud of The AD744 and most truly love the AD825 These are Lightyears ahead of anything BB makes. I also think the OPA-132/134 sound Better than eather the OPA627 or opa-604 Another Nice BB part is the OPA-602 Rather slow but Di-Fet and Smooth sounding. |
aos HeadWizer Joined: Jun. 15, 2008 | Message [#8] posted on: 01-12-2001 09:43 AM CST (US). | | I couldn't hear much of a difference between OPA2604 and OPA2132. However, AD825 definitely sounds different - less warm but reveals more detail. |
| ThingyNess HeadWizer Joined: N/A | Message [#9] posted on: 01-12-2001 10:05 AM CST (US). | | AOS: know of any surplus electronics dealers anywhere around here? I need to find some big surplus transformers, and getting a 30lb unit shipped from the US will cost more than the transformer itself... Oh, and i'm ordering a shipment from www.apexjr.com soon. if you or joobu see anything you need/want from there, lemme know and i'll add it to my order so you don't have to pay shipping. - Thingy |
| aos HeadWizer Joined: Jun. 15, 2008 | Message [#10] posted on: 01-12-2001 04:18 PM CST (US). | | Thingy, Main electronics does sell second-hand stuff (is that surplus?), like oscilloscopes. However, you have to ask them for it as these things go so fast that they're not even put on shelves - you sign up, they call you when they get something, then you buy it (or not). Also, RP Electronics sells A LOT of surplus capacitors, resistors and so on - especially the big store near Brentwood Mall. I got many very small 1uF caps from them for not so much money; as you may know, they're normally notoriously expensive, almost giving Radio$hack a run for their money ;). I'll look at that web site and see if I need anything, thanks. |
| shinew HeadWizer Joined: N/A | Message [#11] posted on: 02-03-2001 10:12 AM CST (US). | | alright, I want to try to use 627 in Cmoy's amp. So I just compared 134 and 627's data sheets today. Offset voltage: Voltage ouput: does that mean I'll need some caps at output as rick described in this tread? headwize.com/ubb/Forum3/HTML/000529.html [Edited by shinew (02-03-2001 at 04:34 PM).] |
| ppl Headphone Council Joined: Jul. 28, 2000 | Message [#12] posted on: 02-03-2001 09:44 PM CST (US). | | Go on just plug in those OPA-627's. Thay will work just fine as a Drop-in Replacement to the OPA-134&OPA-132. I have found that The opa_627 has alot lower offset voltages in real world Circuits than an OPA-134 and about the same as a Prem. Grade OPA-132. My only concern with the OPA-627 is the fact it likes High Supply Voltages. You should also consider the Greater Bandwidth of the OPA-627 that is twice that of The OPA-132 & OPA134 and consider your layout acordingly. I have used Both OPamps in lots of different circuits and can say with comfadence you can do what you wanted as far as using the OPA-627 to replace eather the OPA-132 & OPA-134 as well as The OPA-604. By the way I never Liked the OPA-604, or at least the Dual versions i have tried. |
| shinew Headphone Council Joined: N/A | Message [#13] posted on: 02-04-2001 12:49 AM CST (US). | | ppl, thanks! I'm planning to use 4 627s & 3 9V batteries in this Cmoy. The last time I tried 627 was w/ a single 9V battery in the original pocket amp design. I'm just curious to know how much better it'll sound with a higher voltage(+/-12V) power supply and an added buffer stage. Since I'm pretty illiterate at this, can anyone explain to me(or link me) more about how an opamp's bandwith related to circuit's layout? thanks! |
| ppl Headphone Council Joined: Jul. 28, 2000 | Message [#14] posted on: 02-04-2001 04:03 AM CST (US). | | with one 9 volt as a supply the OPA-627 is about ready to compleatly shut down. The sound will be orders of magnatude better with 3 9volts. you will not evean think it's the same OP-AMP. Everything will get alot better the Bass will get Punchy and quick vs Dull and non dynamic. The mid's and high's will come into focus with mutch better clarity and detail. If you are going to Drive your phones from the OPA-627's output without going through a Buffer like the BUF-634. (whitch you should) then use a heatsink so as to keep Thermal distortion low. The OPA-627 is an older design and the Input transistors are not arranged in an ideal mannor to avoid Thermal upset. But I recomend a Buffer with any OPamp for the same reasons. If a more punchy and Laidback sound is wanted without being Dark or any loss of detail I like The AD-825 alot better than the OPA-627. The AD-825 is the only OPamp that produces the quick articulate Bass i like without any siblence (AD-845) while still capturing the inner details of the mid's & high's. The one quality I like about the OPA-627 is it's Deliation of detail, one of the best i have heard. |
| ppl Headphone Council Joined: Jul. 28, 2000 | Message [#15] posted on: 02-04-2001 04:22 AM CST (US). | | Sorry i forgot to respond to your layout question. Use comon sence and avoid placing the input & output Leads or foil traces close to each other or a supply line. Keep all leads as short as possible, Most importent Keep the (-) Inverting input termanal or feedback components right next to the pin you do not want any lead of foil from this pin greater than 1/16 of an inch without termanaitng into a resistor or other feedback component Also Keep this pin and any foil or lead conected to it away from Gnd.The PC board layout around an OPAMp that i use and has worked for me with any High speed op amp is a Design note from Linear technology # DN50. and while your at the Linear web site I Hightly recomend anyone who builds circuits read the Linear technology Application Note # 47 "High Speed Amplifire Techniques" This is the best treatment on the subject. i have run across Linear technology's website is www.linear-tech.com. then go to applications. |
| shinew Headphone Council Joined: N/A | Message [#16] posted on: 02-05-2001 09:31 AM CST (US). | | Thanks! I just ordered some parts today. Hopefully I'll get it started this weekend. Now I'm interested to know how AD825 sounds. Many people seem to like it a lot. BTW, where can I find AD825s? Digikey & Mouser don't have them. Newark should carry them but they are out of stock. And LC audio doesn't ship their stuff to US... [Edited by shinew (02-05-2001 at 01:31 PM).] |
| shinew Headphone Council Joined: N/A | Message [#17] posted on: 02-05-2001 09:37 AM CST (US). | | never mind, found a few places to order AD825s. |
| rickcr42 Headphone Council Joined: Oct. 10, 1999 | Message [#18] posted on: 02-05-2001 02:17 PM CST (US). | | Ever try to order directly from Analog Devices ? Shipping can be a bit high but if you really need a part ................ Rick BTW-I haunt that website,much good info,Burr-Brown too |
| ppl Headphone Council Joined: Jul. 28, 2000 | Message [#19] posted on: 02-05-2001 05:07 PM CST (US). | | The AD-825 vs The OPA-627 I have investigated Sonicly in my Own Diy Phone amp that operates from 16AAA Ni-Cad's. The Topology uses an OPAMP/ Buffer Combo within A Multiloop Feedback as Discribed by Walt Jung. The Buffers consist of A Paralell set of BUF-634's. The OP-627 was used for about 1 year because it was the Finest sounding Prem. Grade OpAmp available to me> These were in the TO-99 Metal can I fanned the leads (Gold Plate) into an 8 pin Dip pattern. I later ran across the AD-825> I then soldred it into a 8 Pin Dip socket and unpluged the OPA-627 and installed the AD-825. I then interchanged the parts It seeams more often Than I like to have. Upon instlation of the AD-825 i Noticed a Dramitic Increce in Bass Dynamics. It seemed like Having alot more power. The Mids and Highs were Recessed In comparrison to the Bass. The Bass on the Opa-627 also did not seem to extend as deep as the AD-825. The upper octave Extention ( Sony Mdrv6 Phones ) appently reached almost One octave More with tthe AD825 vs the OPa627.Unfortionly Delineation of Detail with the AD825 was Drmaticly reduced. The Prime Halmark of the OPA-627 is the ability to reach deep into the ambient background of most all types of recordings.The Clearity of the AD-825 over the OPA627 is apparent from the male voice upward. The Grit and Grain of the OPA-627 was Noticed by everyone who listened to the amp with the OPA-627. This compleatly went away when almost any other op amp was replacing the OPA-627 I found The AD-825 as well as The AD845 was like a breth of fresh air in comparison to the burr Brown Part. Well got to go I hope this answers some of your concerns. |
| lligior Headphone Council Joined: N/A | Message [#20] posted on: 02-11-2001 11:35 PM CST (US). | | opamps usually give what is called a gain-bandwidth product, this number can usually be found in its data sheet basically to find this number you set the gain of the opamp to 1 and apply progressively higher frequencies, then you find the frequency where the gain is less than one you have the gain bandwidth product, above this frequency it acts as a damper. amazingly it usually works out that if you move the gain to 2 the cutoff frequency is one half of what is was for the gain of one. So if you want to use this number decide your gain, divide gain-bandwidth by your gain and it should tell you approximately what your bandwidth will be |
| ppl Headphone Council Joined: Jul. 28, 2000 | Message [#21] posted on: 02-13-2001 01:45 AM CST (US). | | lligir you have the right idea abuot GBW product. a ballpark aproximation of your closed loop bandwidth or High frequency responce can quickly be determaned from the Data sheet just look at the Open loop gain vs frequency for your device and follow the response line down to whair it intersects with your chosen closed loop gain and then note the frequency of this intersection this will give you a resonable figure less any additional capacitences from layout ect. An example of this if we look at the Data Sheet for the OPA-627 , on page (4) in the lower right hand corner is a graph Called "OPEN LOOP GAIN VS FREQUENCY" notice the gain below 100 Hz. is quite high 120 dB.Now lets say we use the device in a circuit with a gain of 11 (20dB) following the line down we find it intersects the 20 dB gain point on the Graph at just above 1 MHz. This is the Typical high frequency cutoff point @-3dB yout actual circuit is capable of. |
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