Billfort
Addicted Member
I’ve been happily using my Altec 604-8G’s for a few years now but had started to believe the stock Altec crossover was the weak link in my system. I set out to try a bunch of different things including bi-amping but after trying the Markwart design, I’m happy enough with the results that I may stop obsessing on crossovers.
This is a fairly straightforward design using a 12db low-pass and 18db hi-pass section with a later 604-8K/Altec 19 style compensation circuit on the horn. This compensation EQ circuit is supposed to attenuate the midrange energy of the horn while gradually bypassing this attenuation as frequency rises, softening some of the ‘shout’ while complimenting the compression driver’s natural roll-off to extend frequency response.
I’m not sure of the exact x-over frequencies used here and an email from Jeff Markwart didn’t really give me a clear answer but I believe the composite frequency remains at the 1k5 of the stock piece but has the ‘corners’ spaced above and below this number a small amount. This ‘gap’ in the cross points is supposed to again reduce the midrange energy in the 1k5 to 2k area and further reduce the infamous 604 midrange shout.
The prototype pictured here was done to Jeff’s schematic (Simplified Altec Model 604-8H & G network with EQ) as a test mule I could give a serious listen too before committing to all expensive components and a pretty case (that will be external to my speakers). On the recommendation of Bruce Edgar and others, I used NOS GE paper-in-oil motor run caps found on ebay for a few dollars a piece. I formed the 6uF, 12uF and 25uF values needed with selected 6uF values out of large batch along with 1 tin-foil 1uF Solen cap. Coils are Alpha-Core copper foils, resistors low buck wire wounds, a 50w L-pad and wire is old stock Western Electric cloth covered copper. My intent is to replace the resistors with Mills wire-wounds and toss the L-pad after deciding on a value.
The immediate impression of this piece was that it is very smooth in the midrange, a big enough difference that it took me by surprise and had me running through music I had previously ruled out as too ‘in your face’. Initially I thought the treble was a little soft but I think this was just a little midrange shout hangover and things actually seem very extended and sweet now – it’s even more apparent now how some vinyl and SACD’s get things like cymbals so right while CD’s get them SO wrong.
I was a little concerned that the oil caps would make things sound a little dark and slow but this has not been the case at all and they seem to play VERY nicely with the horns. Many probably think ‘audiophile voodoo’ about this next statement but it really seemed that the sound of everything opened up and got ‘airier’ as time went on (burn in ?
).
I don’t know if it’s just a power thing or a subtle shift in system balance with this crossover but I now find myself REALLY liking 2A3 tubes (RCA black-plates currently) and may just prefer them over the 45’s I used previously. A short stint with my old 300B amps was also very exciting as the bass weight and beautiful midrange bloom of that tube really seemed to compliment this new flavor.
I have some more tweaking to do and want to get some frequency response measurements done in the next few weeks to tidy up the loose ends but this crossover was a nice step up for my system and this ugly pile of parts is indeed worthy of a pretty enclosure (pictures to follow).
This is a fairly straightforward design using a 12db low-pass and 18db hi-pass section with a later 604-8K/Altec 19 style compensation circuit on the horn. This compensation EQ circuit is supposed to attenuate the midrange energy of the horn while gradually bypassing this attenuation as frequency rises, softening some of the ‘shout’ while complimenting the compression driver’s natural roll-off to extend frequency response.
I’m not sure of the exact x-over frequencies used here and an email from Jeff Markwart didn’t really give me a clear answer but I believe the composite frequency remains at the 1k5 of the stock piece but has the ‘corners’ spaced above and below this number a small amount. This ‘gap’ in the cross points is supposed to again reduce the midrange energy in the 1k5 to 2k area and further reduce the infamous 604 midrange shout.
The prototype pictured here was done to Jeff’s schematic (Simplified Altec Model 604-8H & G network with EQ) as a test mule I could give a serious listen too before committing to all expensive components and a pretty case (that will be external to my speakers). On the recommendation of Bruce Edgar and others, I used NOS GE paper-in-oil motor run caps found on ebay for a few dollars a piece. I formed the 6uF, 12uF and 25uF values needed with selected 6uF values out of large batch along with 1 tin-foil 1uF Solen cap. Coils are Alpha-Core copper foils, resistors low buck wire wounds, a 50w L-pad and wire is old stock Western Electric cloth covered copper. My intent is to replace the resistors with Mills wire-wounds and toss the L-pad after deciding on a value.
The immediate impression of this piece was that it is very smooth in the midrange, a big enough difference that it took me by surprise and had me running through music I had previously ruled out as too ‘in your face’. Initially I thought the treble was a little soft but I think this was just a little midrange shout hangover and things actually seem very extended and sweet now – it’s even more apparent now how some vinyl and SACD’s get things like cymbals so right while CD’s get them SO wrong.
I was a little concerned that the oil caps would make things sound a little dark and slow but this has not been the case at all and they seem to play VERY nicely with the horns. Many probably think ‘audiophile voodoo’ about this next statement but it really seemed that the sound of everything opened up and got ‘airier’ as time went on (burn in ?
).I don’t know if it’s just a power thing or a subtle shift in system balance with this crossover but I now find myself REALLY liking 2A3 tubes (RCA black-plates currently) and may just prefer them over the 45’s I used previously. A short stint with my old 300B amps was also very exciting as the bass weight and beautiful midrange bloom of that tube really seemed to compliment this new flavor.
I have some more tweaking to do and want to get some frequency response measurements done in the next few weeks to tidy up the loose ends but this crossover was a nice step up for my system and this ugly pile of parts is indeed worthy of a pretty enclosure (pictures to follow).
