Upgrading the Dahlquist DQ-LP1

Bob

Super Member
The Dahlquist DQ-LP1 is an active crossover device that provides high and low outputs for bi-amping your system. The preamp output goes to the DQ-LP1 and its high output goes to one amp/speaker combo and the low output to another amp and speaker combo. You can set the crossover frequency from the front. In case you have different amps for both, you can set the level for the bass. I have a SE tube amp for the top and a Hafler DH-200 driving a set of Monitor 7Bs for the bass (yes, I know its full range but it works for me).

The theory is that instead of having one speaker go full-range, both high and low, you assign two speakers to cover the two frequencies often allowing each to work better. The other side of things is interesting. Most bi-amping comes from home theatres where sub-woofers are used to play the opening scenes from Top Gun where you can rattle your neighbor’s eyeballs from a mile away. For you bass-heads, there’s left, right, and dual center-summed outputs for rattling teeth in your area code. Just imagine 4 1000w sub-woofers...

But the original intention was to add warmth and extend the bottom. The reality is that at low volumes the bass still comes through. Almost the equivalent of using a loudness control at low volume to boost the missing bass. Nowadays, that loudness switch is a hit or miss.

So for me bi-amping is more than the sum of the two. I get better highs and extended bass. The mini-monitors by themselves sound great but is missing bass umph, the 7Bs are Ok full range but without the mid-range magic. I get both with bi-amping.

I’ve had this for decades and a while back the left low output went AWOL. Simply missing. I seem to have left channel problems. First my master DH-101 had noise in the left channel phono. The other Hafler DH-101 I have had an intermittent left output. The DH110 had a normal left channel but low volume in the right.

I suspected a bad IC but the effort to fix this was high enough for me to postpone tackling it until late last year.

I needed a schematic and the ones on the web needed better resolution to see the parts values accurately. Lucky I had had a subscription to Van Alstine’s Audio Basics and the schematic there was readable. The next problem was the parts layout – there wasn’t one. So I was down to replacing parts til it worked (yes, I know I should have signal traced the circuit and did this and did that and I did do some of them).

The parts in the unit were circa 1975 or so. The caps - polyester like the suits BUT HP of the Absolute Sound used this unit (with Van Alstine mods – which I’ll get to later) to create his Frankenstein – the QRS/1D. It was an Infinity and a Magneplanar hybrid (no batteries though).

The next set of issues was that the ICs (4558s) were soldered directly onto the board so I needed to remove all the ICs, add sockets, buy some 4558s in case one of the ICs were bad (and one of them turned out to be bad).

Then I thought there might be an RCA connector problem.

So, between all these issues I decided to do a ground-up restore like the car guys do.
 
Here’s the plan

1. Upgrade the caps to improve the sound – signal path polyesters and power supply electrolytics

2. Add IC sockets for ease of repairs and possible upgrades

3. Replace the carbon (film?) resistors with metal films for lowered noise floor

4. Upgrade the power transformer to allow more current for possible IC upgrades

5. Replace the RCAs in case there are problems there

Here’s a picture before starting major surgery – the two FAT blue caps on the left are from the Van Alstine mods.

0-inside-original.JPG

Caps to be replaced are all the black polyester caps plus the orange-topped caps. And not to forget all the electrolytics (mostly in the power supply).
 
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Here’s a picture of the caps to be replaced.

1-DQ-LP1-caps.JPG

This was after I removed the soldered in ICs (5) and added sockets. I saw visions of upgrades and SQ improvements. And maybe an Oscar for best performance with a bag of ICs.

I labelled each of the older polyester caps to order and eventually replace.

There’s two light blue caps (also polyester) at the bottom center and bottom right that was wired in to provide impressive/accurate impedance mating of the high pass output with the next-in-line amp.

You will notice there are two orange-topped rather large caps. These are Mullard (yes, of tube fame from their legendary, storied, universally-hailed XF series of EL34 tubes) polyester high-voltage caps that are referred to as tropical fish. These are highly-valued in boutique guitar applications. 2uf.

Some stats. The Power Supply is +/- 15V. The fuse is fast blow 1/32A (32ma) and is 3AG (6.3x32mm) and there’s a whole story behind this but I’ll save it for later.
 
Let’s look at the caps list – signal path first

Qty ID value new

2 C6, C10 .015 orange drop – CDE 715p15352JD3
2 C4, C9 .018 light blue – EPCOS/TDK B32641B0183J
4 C1, C7, C8, C11 .027 yellow polypropylene
2 C5, C12 .15 Black – Panasonic ECW-FE2J154J
2 C2, C14 2 Black – Panasonic ECW-FE2J225J
2 C3, C13 100/10P recommend P or NP @16V or 25V

Others – power supply

3 C15, C16, C17 47/25P 47/50P or NP
2 C18, C19 220 P original - axial
3300/25P Van Alstine mod / 3300/40P used

2 C20, C21 2000pf ? replace with micas ? I didn’t

Now a look at the resistors. Do check my list (from my notes) against your unit’s insides.
(some purists will complain about the musicality of CF vs MFs but I always prefer a tighter tolerance and lower noise floor).

QTY ID Value New

3 R6, R17, R22 47
4 R3, R5, R24, R291K
1 R26 2K dropping Resistor for LED
1 R13 3.3K
1 R16 4.7K
2 R2, R23 8.2K
11 R7-9, R11, R12, R14
R15, R18-21 10K
2 R1, R25 33K
2 R10, R27 100K
2 R4, R28 470K

Power supply probably 1/2W if replaced – I didn’t

2 R30, R33 3.9K
2 R31, R32 15K


Also, five UF4007 for the power supply and the LED (optional unless you think the SQ improves)

Please verify the list above with your copy of the schematic – check off each on the schematic to ensure that I got it right. 29 Resistors plus an optional 4 for the power supply.
 
A couple of notes on the caps I used

C2, C14 the 2Uf cap: originally the Mullard tropical fish was replaced with the Panasonic ECW-FE2J225J which is black. It is a polypropylene unit that is intended to provide spooky black bass notes that stops your heart with a bass thwack.

C1, C7, C8, C11 are .027 polypropylene units I had lying around. Again selected for the yellow contribution to the airy highs. I use these as bypasses elsewhere.

C5, C12 are .15 black caps from Panasonic for quiet support of upper mid-bass to preserve the boogie line

C6, C10 are .015 Orange Drops for that mid-range wow – you duck when the third violinist sneezes.

C4, C9 are .018 light blue TDKs for that cloud interface that transcends hard reality.

But seriously, you will not see many Wimas in my stuff. While it is red and adds fire to the sound, it is the world’s most counterfeited cap (just behind Alps pots with their cheesy san serif fonts that neon flashes “fake”). So, how do you know it’s a real Wima – provenance is the difference. Buy from major distributors and toss the fake reds.

Here’s the original Mullard tropical fish 2uf polyester cap and its Panasonic ECW replacement. It’s a bit bigger and barely fits so don’t go larger. This is used as a high pass filter.

2-DQ-LP1-high-pass-cap.JPG
 
If I were a newbie and like cars, bigger is better, I would be looking at replacing the Mullard tropical fish caps with a real man’s cap – the Obbligato oil 2uf (but I wouldn’t be able to put the cover back on):

3-DQ-LP1-high-pass-obbligatos.JPG
 
Here’s the left side after cap upgrade with the tropical fish replacement waiting til later.

4-DQ-LP1-new-caps-left.JPG

And the right side:

5-DQ-LP1-new-caps-right.JPG
 
Next up is the Van Alstine mods for the DQ-LP1.

This mod added a bunch of 620pf silver mica bypasses across the caps and is typically added on the underside:

6-dq-lp1-pre-mod-bottom.JPG

And massive caps in the power supply:

6-large-PS-caps.JPG

And in upgrading the power supply caps from 3300/25 (left, original) to 3300/40 (right, new):

7-DQ-LP1-PS-cap-size.JPG

If you’re interested in this upgrade contact Van Alstine or search for this on the web.
 
Some last minute notes before we tackle the really tough problems I found myself trying to solve.

If you socket the ICs, do mark the orientation, pin 1, with a marker on the board that will still be visible after installing a socket.

Facing the front and viewing front to back, the left three 4558 have their pin 1s facing the rear (and the IC printing readable when facing the right side). The middle two 4558 with the .015 cap in the middle, both have pin1 facing the front. The 1468 VR chip is at the far right and is not replaced or socketed.

DO VERIFY THAT YOUR UNIT MATCHES THIS DESCRIPTION OF THE IC ORIENTATION. ONE OF US IS WRONG IF IT’S DIFFERENT. DO NOT PROCEED UNTIL THIS IS RESOLVED.

First set of problems

As you know, there have been dozens of new opamps since the 1970’s when the 4558 was used in the DQ-LP1. There have been some who believe that the DQ-LP1 would sound better with more up-to-date opamps. I thought so, too. Until I realized the power transformer could not supply enough current to support five replacements for the 4558.

I tried to upgrade the transformer (original on the left):

8-DQ-LP1-trafos.JPG

The original transformer is a Signal Transformer model DPC-34-35 rated at a super low 1.2VA with dual primaries, 34V CT secondaries at 35ma. No sense in naming replacements since:

As you can see, the original transformer is an exact fit and the two potential replacements are either too tall (blocks the cover re-install) or too wide and intrudes on other components. So back into the unit went the original transformer and the dreams of 2604s, 5532s, 4562s, gone forever. Not going to re-engineer the unit. But socketing the ICs is still a good idea.

Good news, the 4558 is still available and there are rumors of updated 4558s for higher performance whatever that might be. Maybe I’ll try them later.
 
The next problem are the RCAs. It took me a long time to remove the angled RCA connectors mounted on the main board. You have to stick your soldering iron underneath the rear lip. Then you have to mount the new RCAs to the back panel and wire them back into the main board. Dumb move on my part.

9-DQ-LP1-connectors.JPG


Next set of problems: since these were already in my bag of parts I had to fit these new RCAs into the existing unit. The outer mounting disk has an inner lip that was a little small for the holes in the DQ-LP1 back panels ( about 1/2 inch) so there’s a little slop. But once the connector is attached there isn’t the possibility of falling out.

Wiring the RCA socket back to the main board was painful as was the ground signals.

10-DQ-LP1-connectors-removed.JPG

And finally, inside:

11-DQ-LP1-inside-new RCAs.JPG

and outside

12-DQ-LP1-rear-new.JPG

Do note the very large 2uf (black Panasonic PP) under the front lip on the left side – barely fits. And the other Orange Drop under the front lip on the right.

If I had to do over again, here’s what I would do. I would pull the board out of the unit, remove all the RCA connectors – much easier this way. Then I would fab a double strip of transparent material, measure the mounting diameter of the replacement RCAs and drill eight holes. Then make eight circular disks of less than the back panel hole diameter with a hole in the middle just larger than the RCA inside mounting diameter for a loose fit. These disks will allow mounting the RCAs to the outer strip and the inner strip without flexing the dual strip. And should be stable for all time. The inner disks prevent movement and insulates the RCA from the back panel.

And finally to the very last problem.
 
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Fuse holder blues

For all you guys with older equipment, hopefully newer than this late 1970’s unit, pull your fuses as a test. I pulled the fuse on this almost 40 year old unit and the metal endcap of the fuse remained stuck inside the fuse holder cap. Lost this fuse that lasted 40 years.

Then the insult on top of injury is that all my fuse holders are ½ inch and the DQ-LP1 fuse holder is only 12mm. ½ mm too small. So I bought a fuse holder but it didn’t come with a cap. And the next order’s cap didn’t fit with all the fuse holders I had. Then I contacted one of the fuse holder mfg for a “sample”, their VICE PRESIDENT told me the part numbers to order. Thanks Mister VP.

and yes, I could have filed away .5mm but with luck, the shavings I missed picking up would come back to haunt me, especially at a time I forgot I did this and with eyes that much worse for wear and tear.

I finally got the proper fuse cap for the now fitting fuse holder. I put in a 1/32A fuse and promptly blew two in a row. Maybe some of my passive component upgrades is now hungry, thirsty, and mean. I put in a larger fuse and all is well.

It’s back in the system and bi-amping like a champ.

Here’s a final shot of the insides:

13-DQ-LP1-inside-updated.JPG

I am grateful for all the Akers that contribute their knowledge, help, advice, and generosity. I hope that this will help someone else out there.

And if there are any mistakes, omissions, I apologize for them and please feel free to correct and comment on the content in this thread.

Bob
 
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Since I just put my LP1 in the system after having it for a few years, I have read the manual a few times, made some mods and so forth. A couple items about this nice rebuild how to.

The unit is called a continuously variable low pass filter because it has only one capacitor in the high level output and no controls of that output other than bypassing the low out to provide a combined signal to the high out (allowing comparison of the with and without sub sound of the main speakers). For the reason that only C1 and C8 are in the high level signal acting at the high pass filters, I think these 0.027µF caps should be the best audio cap you can find. The 2µF caps are not the HPF caps. They are at the input to the low pass section.

Although discussed on the manual on the Yahoo group (I think). I don't know how hard one needs to work to change the ICs since they operate on the signal to the sub(s). If used with large main speakers with response down to say 100Hz or below, would an IC change make a big difference? I know the unit as configured at the factory was able to provide signal to the low output front panel adjustable from 40-400Hz and down with the HP at 60-80Hz, depending on high level amp input impedance and internally adjustable with parts changes.

I like that you went with the pp caps as that is what Dahlquist recommended along with the 620pF silver mica bypass caps on the unit.

It seems the only mod not mentioned in that manual is the bump from 220 to 3300µF for the PS caps. I have a pair of ~2000µF units I might just throw in there since I used something else when those were needed.

Well done, glad to see it at this time. Maybe I'm not done poking around inside mine, yet, thanks.
 
thanks for the correction

I think I meant low pass filter. but yes. There's still sound going out to the high outputs even if the unit
is turned off.

As I mentioned earlier this unit cannot support other op-amps due to the transformer's low current rating.

The transformer is only good for 35ma. Each 4558 uses 5ma. As an example, typical Iq for the
OPA2604 is 10.5ma for each chip. the 5532 is also over the transformer limit.

However, it's a moot point until you dive in an replace the soldered 4558s with sockets.

the bigger problem is that the DQ-LP1 uses some of the op-amp sections as buffers. As you
know some op-amps are not unity gain stable or stable if the gain is >4 or 5. so willy-nilly
op-amp rolling is a careful exercise of datasheet notes.

the 620pf silver micas were van Alstine's idea - genius if you ask me - he misordered thousands
of these and created a huge demand for them in the DQ-LP1 upgrade kit.

Jon Dahlquist did suggest the use of MF resistors as an upgrade as well as careful selection of 4558s
for lower noise. I recall seeing special 4558s that are low noise.

the bump from 220uf to 3300uf is part of the van Alstine upgrade. The ones I replaced were huge and
the replacements could have been smaller had I used lowered voltage ratings.

There is definitely a sound quality improvement. my reference (this word make it sounds expensive
but it isn't) uses this in bi-amp mode and occasionally I listen to the 7Bs in full-range but only for a few minutes -
it's nice but I hear better sound and prefer the bi-amped mode.

I think between the van Alstine mods and changing out the polyesters for PP, you will hear a big difference.
(the only bigger difference was modding the DH-101 and the LS3/5a). BTW I couldn't figure out how
to use a standard BLUE LED to replace the nipple-head red LED.

Bob
 
To bring this back from the dead, I'm re-building one now. This was a great crossover that I used for years until I bought my REL T-5's. I retired it when it lost a ground on a bass channel and blew up a sub. I'm now going to use it in a second system with custom sub/sat speakers. I've replaced all the RCA's with gold panel mounts like above, and all the electrolytics with Nichicon Muse and Fine Gold caps in the stock values. I have Rel Cap polystyrene film caps and metal film resistors for the whole thing, but for now I'll just do the .027 cap in the high pass. I got all the parts at Michael Percy Audio a while back. I'll let you know how it works out.
 
excellent!

only two cautions. first, is be very careful about the size (length and width, lead spacing) and
the ability to fit in the allowable space - not just board real estate but height. try it before you
unsolder the old component.

second, polystyrene has a low melting point - think Styrofoam cups, so use lots of
clip-on heatsinks on the cap's leads on the top side (parts side of the PCB).

as I mentioned earlier, this can be one of the sound quality improvements to your system.

lastly, you may/may-not need a subwoofer. use floor standers since they all go down low.
in my case, subwoofers (at the time) didn't go up high enough to cover the missing low
end of the treble speakers.

good luck and post or pm any questions. most of any issues I encountered are above.
do note that the user manual has a number of calculations for freq response and loading
based on the circuit's original (cap) values so keeping it the same is good.

also note I needed to up the fuse value, you may need to, too. I think I used 1/10A.
 
Thanks Bob. It was a tight fit with the panel mount RCAs and where the new caps went. I had to offset them from the holes quite a bit to get it all to fit. The REL's are pretty big, and it took two to get the .027uF. I'm using this with a pair of Vifa 6-1/2" sealed monitors I'm building mostly because I already have the drivers. I also have a pair of really good, really old, home made 10" passive subs. I'm planning to run this with a Rotel RX-855 receiver and a RB-850 power amp in mono to drive one sub, also because I already have them. The system will be in a small loft space, so one sub should be plenty. The Rotel receiver has pre-out/main in jacks. Right now I'm just trying to find the input impedance for the power amp section of the receiver, because I can't find it published anywhere. It's 27K for the RB-850 power amp, so maybe it's the same?
I used this crossover for a long time with my DIY Lynn Olson Ariel TL speakers from the 90's and a pair of the aforementioned subs. It sounded great with an ARC pre-amp, modded Dyna ST-70 on the Ariels, and an Adcom triple nickle driving the subs. Domestic demands required smaller subs, so I'm using a pair of REL T-5's on the Ariels now.
 
Well, it works. I hooked it up to the Rotels and used an old pair of Celestion 1's for the satellites. I assumed 27K for the input impedance, and it sounds like that was about right. It all sounds great so far. Now for some listening and perhaps a wee dram of single malt. Tomorrow it's time to start building speakers! Thanks again Bob for all your tips and insight.

Cheers

Phil
IMG_0018.JPG IMG_0016.JPG IMG_0013.JPG
 
congrats! and excellent that you made it all work.

note the DQ-LP1 can use a single (combined signal) bass, duals and more - I forget the number
so if you want to blast the opening scenes of "top Gun" you can.

BTW the one very nice thing about running bi-amped systems is that when you lower the
volume the bass stays strong and doesn't disappear.

enjoy the music.
 
Dear bob,
Thank you for all your work on the Dahlquist crossover for us. I have one that has a fair amount of distortion in one channel. What would be the down side of a completely separate power supply to feed the OPA2604's, and would that be your best choice. Is it pin for pin or are other adjustment need to be made. Quieter is always better. What transformer did you buy that didn"t fit? Is a 2 amp unit enough.
 
I wish I could pull the circuit board out of mine. I have one grub screw on a knob that will not come off. I'll try again sometime as it is soaking in pb blaster now while the unit is in service. If I need to get inside for some reason, I can always drill out the screw and replace it. I'm not that serious about getting the board out at this time.
 
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