Not thrilled with sound of 4562 in active xovers

[Michael Bean]

The ESP boards were originally designed for JFET input opamps and the LM4652 is a BJT input opamp, if you didn't scale the circuit compnent values to account for that, it might be part of the problem. The 4652 works best if you keep the resistor values somewhere between 2k and 5k. I've also found they need a little more care with power supply decoupling. I put low ESR 22uF caps as close to every power supply pin as physically possible and I also put 10nF NPO discs on bottom side of the board directly on the pins, and don't use IC sockets, solder the opamps directly to the board. Additionally, I use a 10 ohm metal film resistor between the power supply rails and the decoupling caps. Beyond that, just guessing about proper crossover point is hit and miss, usually miss. The natural 2nd order high pass response of the drivers must be accounted for, using stock LR24 filters won't acomplish that, so I'm not surprized the results you got were disappointing. You really need to measure the drivers in the box to get it right.

Mike

 

[HornTube]

Bypassing caps is like playing roulette, most of the time you loose...
The first thing that gets hurt are the highs and the upper mids, they do get separated from the rest of the musical spectrum and other tonal weirdness in conjunction with the type of material used in the caps.

 

[Michael Bean]

HornTube, I don't randomly put decoupling caps in my designs. I design on paper, measure and scope for proper performance, listen to the results and go back and tweek if necessary. I spend a lot of time getting these things right. Every opamp must be utilized properly and have it's shortcommings identified and catered to.

Mike

 

[Dave S]

Thanks for all your suggestions.

Dave, that board you linked certainly is more complex than the ESP design, but I'm not sure I'm interested in the extra features. What makes you say it has better engineering and layout? Unlike the ESP site, they tell you almost nothing about the circuit or design. Value is better with on-board regulation, but I am already kind of committed to the ESP boards, what with a power supply and all the xover parts. The ESP board also has balanced input, which is a nice feature for me.

Regarding parts selection: I am using WIMA poly film caps and PRP resistors at all signal path locations. I think those are very good parts, and I suspect they don't leave too much of a bad sonic signature. I have built monoblock amps for the mids/highs with one half of a stereo xover inside the same enclosure for each channel, so there are no IC's involved. Internal wiring is OCC solid core copper/teflon.

The PS is decent quality: it was used in the Hagerman Clarinet preamp. Right now I am using one for both xovers with an umbilical running between them. I plan to build another supply so I have one for each xover, but I expect that will not greatly affect the sound.

The opamp power supply is bypassed by Panny FM's, small ceramic caps on the PCB, and a ceramic cap soldered directly across the power pins of each opamp. The lead length is about as short as it can get!

I am currently considering reconfiguing the xovers as 3rd order Butterworth to find out what effect it has on the lower mids/upper bass.

Peace,
Tom E

It seems that you know what you're doing. I don't really have any other suggestions as to why the result is not great. Like others have said you really need to look with a 'scope in case something is amiss.
The main reason I like the Audio Kit pcbs is that they have a proper ground plane. I also checked the sound out first, by using the BSC opamp as a preamp (2X gain using input opamp then the pot then the BSC opamp wired as a buffer) and it sounded better than all my other preamps - and I've built a lot!
I guess AudioKits could provide more info but all the circuits on the pcb are totally standard text book stuff.

Just one thought...
When you get more/deeper bass it is often neccessary to add a bit more to the top end to get a balanced sounding system. This is why small speakers like LS3/5As have tailored hf response.

Oh and another thought...
Don't waste money on OPA627s (which are single opamps anyway). IMHO they are good hifi but lose the music - although I have no idea how they can possibly do this

 

[madisonears]

Horntube, are you responding to this thread? I don't see anyone writing about bypassing (paralleling) caps, except across the PS rails. This is common practice, required for most (all?) opamps. I didn't mean using two caps in parallel in the signal path. I agree that is a bad practice, and I rarely do it with line level signals. Even paralleling 'lytic/film can give weird sounds, and I try to avoid it. Passive xovers are a different matter, and a subject for another thread altogether.

Dave, thanks for your input about OPA627. I've seen many posts that claim it's the best opamp available. The cost is prohibitive for me, so I really can't consider it. Besides, I believe these other posters who state that the opamps are probably not the cause of the midrange hollowness.

Michael, I like your suggestion to "scale" the components and use smaller resistors. Now the circuit contains 10k and 6.6k with .47uF caps. Perhaps I'll try smaller resistors with bigger caps. I don't like or use opamp sockets.

I will try different slopes to see if that helps.

If anyone can please help, I have one technical question regarding output configuration. It's a common topic, but this might be a unique situation. The xovers have balanced input and single-ended output. My mid/high amps are single-ended, so that's all fine.

However, my bass amp is balanced input ONLY. Right now, I am using the single-ended xover output to pins 2 & 3 of the XLR amp input. There is very low level hum, and it doesn't really bother me. I tried hooking pin 1 of the XLR xover output plug to pin 1 of the xover XLR input plug, but that didn't improve anything, so now pin 1 of the XLR output is not connected to anything. There is an output coupling cap on the ESP board. It is on the positive leg of the single-ended circuit, which goes to XLR pin 2. Do I need to put another equal value cap on the pin 3 negative leg because I am running this to a balanced input?

Peace,
Tom E

 

[Michael Bean]

Take a look at this - Single ended to balanced connection | AVguide

Mike

 

[madisonears]

Michael, thanks but I've already got that part done. Pin 2 of XLR is hot, pin 3 is ground of single-ended signal.

Maybe my question was too wordy. Condensed: if there is a xover output coupling cap on the hot leg going to XLR pin 2, must I also have an output coupling cap on the ground leg going to pin 3? Normally, this would not be required for a single-ended connection, but I am going to a balanced input. Does that change anything?

Peace,
Tom E

 

[Michael Bean]

No don't use a cap on pin 3 to ground, it needs to be a dc connection, In other words just short pin 3 to pin 1.

Mike

 

[stoc005]

I think it could be the drivers and the XO frequencies you have chosen. Have these drivers been used with those XO frequencies before? Can you listen to a comm. speaker with the same drivers to hear how it sounds?
You have to have some measurement capability at some point. I use a "flat" RS SLM and a warble tone CD. It works well enough in the lower midrange and above to find out that my tweeter was 1-2 db too loud overall. The clue was that the speaker lacked bass and the sound was being overwhelmed by the tweeter level. Got the mid and the tweeter polarity correct? Simple to get it wrong. Lots of good ideas so far but op-amp "sound" may not be it.

 

[ClaveFremen]

Hi Tom,

I am using the ESP 24db/octave boards, populated with quality parts, including LM4562.
...
With the same amps and xovers in the circuit, there is plenty of detail (almost too much), but the midrange seems fake, with kind of a shallow or hollow tonality. Some of that may be due to xover point/slope issues, and I think I can fix that. But the highs seem "flat" and somehow truncated, with no sign of life or openness, and that is NOT a slope or xover point problem.

LM4562/LME49720 can be a very good sounding opamp but in the wrong place or not correctly decoupled it can sound 'gray' or even harsh.

Wrong places are I/V conversion and output buffer... very very good for filtering, summing/unbalancing, etc..

It has limited current output capabilities so it couldn't be good as last opamp in the chain... it needs a buffer after it.

In addition to the designed bypass caps, I have added ceramic caps soldered right across the power pins of each opamp. I don't think that's an issue.

It's not an issue but I would try a 10nF FKP2...

I am on a limited budget, so I can't just buy 8 OPA627's or discrete opamps, unless that is absolutely the only alternative to achieve good sound.

If I were to try a couple 627's, would they yield the best results as input buffer, output buffer, or in the filter section?

Instead of OPA627 I would try the newer OPA827 which is cheaper and, IMHO, better. You can find it with very good prices on eBay.

If you decide to try OPA827 bypass it rail to rail with the 10nF FKP2 and 47uF rail to ground (Cerafines).

 

[madisonears]

Dario, thanks for the opamp suggestion, but those are surface mount, and I want thru-hole. I do not want to get into adaptors.

Maybe I'm just lucky, but changing the high/mids filter to 3rd order Butterworth, with reversed phase, has completely transformed the sound. Low pass is still 4th order LR, and the slopes seem to mate really well. I don't know whether that's proper or not, but the sound is gloriously full and detailed from the top end right through the midrange, now VERY present, down to the bass.

There might be a better combination of slopes and signal manipulation to be found with thorough measurement and knowledgeable design, but this sounds really good to me. This is why I bothered to build my own speakers.

Haven't messed with the balanced connections yet, but I will.

Peace,
Tom E

 

[ClaveFremen]

Dario, thanks for the opamp suggestion, but those are surface mount, and I want thru-hole. I do not want to get into adaptors.

As always, you're welcome Tom.

BTW you miss something... also LM4562, IMHO, is better sounding in the SMD version...

If you want thru-hole you can try OP275 as last opamp so you avoid problems with the current challenged LM4562